API integration is the work of connecting one software system to another via their application programming interfaces (APIs) so they can exchange data automatically. Two systems that previously needed a person to copy data between them now talk directly. Less rekeying, fewer errors, faster cycles.
Almost every modern SaaS product offers an API: Xero, QuickBooks, HubSpot, Salesforce, Stripe, Microsoft 365, Google Workspace, Shopify, Mailchimp, Klaviyo, DocuSign, Twilio, and thousands more. Integration is what turns a stack of disconnected tools into a single working system.
Examples we ship regularly: a Stripe-to-Xero integration that posts subscription payments straight into the accounting system. A HubSpot-to-Slack integration that pings the sales channel when a deal hits a stage. A Shopify-to-Klaviyo integration that builds segmented audiences from real order data. A custom job-costing app integrated with supplier APIs for live materials pricing. A booking system integrated with Google Calendar and Outlook for two-way sync. Each integration is a specific job: read this, transform it, write that, handle failures.
When two systems hold related data and a person is currently moving it between them. When the volume justifies the build (manual handling of fewer than 10 records a week may be cheaper to leave alone). When both systems offer reasonable APIs (not all do; some "no-API" legacy systems require workarounds). When the integration is durable enough to be worth maintaining (one-off data exports do not need integration; ongoing data flow does). Fixed-quote bands: £1,500 to £6,000 for a focused single-direction integration, £4,000 to £15,000 for multi-step or two-way sync.
When one of the systems has no usable API and no maintained third-party connector. When the data shape is changing fast enough that the integration will break monthly. When the volume is genuinely low (one or two records a week, where automation costs more than the manual work). When a no-code tool like Zapier or Make would do the same job for a fraction of the build cost. The discipline is to integrate only what the business actually needs, not everything that could in principle connect.
Digital Signet API integration work is fixed-quote, scoped before work starts, and built to be maintainable. We use Zapier and Make where they fit (no point building custom when a no-code flow does the job), and write direct integrations when volume, complexity or cost justifies it. The wider context is on the app builds page. The closely related work of stopping manual data movement between systems is on the Sheets automation guide.
Got two systems that should talk to each other? Tell us the data flow and the volume. We will tell you honestly whether a Zapier flow, a Make build or a custom integration is the right tool.
Email oliver@digitalsignet.com