APIs Every SaaS Platform Needs: Stripe, Xero, Twilio and Companies House

A SaaS platform without integrations is a closed system. Data sits in silos, manual steps accumulate, and customers outgrow the platform the moment they need to connect it to the rest of their business. The SaaS platforms that retain customers long-term are the ones that integrate deeply into existing workflows — accounting systems, payment providers, communication tools, identity verification. This guide covers the four APIs that form the foundation of a well-integrated UK SaaS platform, why each matters, and what to build with them.

Stripe API — subscription billing and payment infrastructure

For any SaaS platform charging customers for access, Stripe Billing is the most complete subscription management solution available. It handles trial periods, plan upgrades and downgrades, proration when customers change plan mid-period, failed payment retries with configurable dunning sequences, customer self-service via the Stripe Customer Portal, and revenue recognition across multiple billing models — flat-rate, per-seat, usage-based or tiered.

The Stripe API handles the full payment lifecycle through webhooks. When a subscription is created, a payment succeeds, a payment fails, a customer cancels or a trial expires, Stripe fires a webhook event to your server. Your integration listens for these events and takes appropriate action — provisioning or revoking access, sending notifications, updating your customer record, triggering an offboarding sequence. A well-built Stripe integration means your platform’s access control is always in sync with payment status without any manual intervention.

Stripe Radar provides machine-learning fraud detection on every payment, included in standard pricing. Stripe Connect is available for marketplace or multi-tenant SaaS platforms where you need to collect payments on behalf of sub-accounts and distribute funds. Stripe’s UK transaction fee is 1.5% + 20p for European cards — competitive for a payment infrastructure that requires no monthly fees on the standard plan.

For UK SaaS platforms, Stripe is almost always the right starting point for payment infrastructure. The documentation is the best in the industry, the sandbox is realistic, and the webhook system handles the full subscription lifecycle with precision.

See the Stripe API integration service and the Stripe API integration guide.

Xero API — accounting automation and financial visibility

Accounting is the most common manual overhead for SaaS businesses after payment processing. Invoices need to be raised, payments matched against them, bank transactions reconciled, VAT calculated and reported. Without integration, this means exporting data from your platform and importing it into accounting software — a monthly exercise in manual data entry that scales poorly and introduces errors.

The Xero API connects your SaaS platform directly to Xero’s accounting engine. Invoices are created in Xero automatically when a subscription is created or renewed. When Stripe confirms payment, the corresponding Xero invoice is marked paid. Failed payments create credit notes. Refunds trigger the appropriate Xero entries. The result is accounts that are always current, reconciled and audit-ready without anyone touching a spreadsheet.

For SaaS platforms, the Xero integration should also connect to Stripe subscription events. A Stripe subscription renewed today should create today’s Xero invoice. A Stripe refund should create a Xero credit note. A Stripe payment failure should mark the Xero invoice as overdue. The integration connects two systems that otherwise require manual bridging — Stripe and Xero — into a single automated financial workflow.

Xero uses OAuth 2.0 authentication with a multi-tenant structure that supports connecting multiple Xero organisations to your platform. For SaaS platforms where each customer has their own Xero organisation, the multi-tenant OAuth flow allows customers to connect their Xero to your platform directly.

See the Xero API integration service and the Xero API integration guide.

Twilio API — transactional communications

Email open rates for transactional SaaS notifications hover around 20–30%. For time-sensitive communications — login verification, payment failure alerts, security notifications, trial expiry warnings — email is a poor delivery mechanism when the message needs to be seen immediately. Twilio SMS delivers messages directly to the recipient’s phone, read within minutes.

For SaaS platforms, Twilio covers three critical communication use cases:

SMS two-factor authentication via Twilio Verify. Users logging in or making significant account changes receive a 6-digit code by SMS. Twilio Verify handles code generation (cryptographically secure), delivery, expiry, rate limiting and brute-force protection. This is meaningfully more secure and faster to implement correctly than building OTP logic from scratch. For SaaS platforms holding sensitive business data, two-factor authentication is increasingly an expectation rather than a differentiator.

Transactional SMS notifications. Payment failed — card about to expire — trial ending in 3 days — invoice ready to download. Messages that need to be seen immediately are significantly more effective via SMS than email. A payment failure SMS with a direct link to update payment details gets dramatically higher response rates than the equivalent email, which directly affects subscription revenue retention.

Phone number verification. Verifying a user’s phone number at registration confirms they own the number they’ve provided, prevents fake account creation and provides a verified second factor for future authentication. Twilio Verify handles the verification flow in the same product used for 2FA.

See the Twilio SMS API integration service and the Twilio SMS API guide.

Companies House API — business verification and enrichment

For B2B SaaS platforms selling to UK businesses, Companies House data provides instant business verification and enrichment at registration or onboarding. When a prospective customer enters their company name or number, the Companies House API returns: the registered company name, number, registered address, incorporation date, SIC codes (business type), company status (active, dissolved, in administration) and director information.

This data serves several valuable functions for SaaS platforms:

KYC-lite verification. Confirming that a business is actively registered with Companies House, incorporated in the UK and not in administration provides a basic level of business legitimacy verification without full KYC processes. For SaaS platforms offering credit terms, free trials or high-value features, this provides a lightweight check before extending value.

Onboarding data pre-population. When a business customer registers, requiring them to enter their company name populates their registered address, company number and legal name automatically. This reduces registration friction and ensures the data on file matches the legal entity — important for invoicing, contracts and VAT compliance.

CRM enrichment. Company data retrieved at registration populates your CRM with firmographic data: company size (inferred from employee count in filings), industry (from SIC codes), age (from incorporation date) and location. This informs segmentation, pricing strategy and sales prioritisation without manual research.

Ongoing monitoring. The Companies House API can be used to monitor the status of existing customers. A customer whose company enters administration is at risk of churning or defaulting. Monitoring key customers’ Companies House status provides early warning of potential issues.

The Companies House API is free to access with an API key issued via Companies House’s developer portal. Rate limits are generous for normal business applications.

See the Companies House API integration service.

The integrated SaaS stack

These four APIs are not independent tools — they form a connected workflow. Stripe handles payment and subscription lifecycle. When Stripe confirms payment, Xero creates the invoice and marks it paid. When Stripe reports a failed payment, Twilio fires an SMS to the customer. When a new customer signs up, Companies House verifies their business and pre-populates their account. The four integrations combine to create a platform that operates efficiently without manual intervention at every stage of the customer lifecycle.

For a complete integration service across all four APIs, or any individual integration within this stack, see the API integration services overview.

If you need help with a custom API integration for your website or business application, I can help. I build bespoke API integrations for UK businesses. See API integration pricing or get in touch to discuss your project.

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