Signing a document online is easy in a demo, tricky in production. The subject is states and webhooks, not the sign button.
A real-estate, HR or legal product needs to get documents signed. E-signature APIs (Yousign and equivalents) handle legal value. The integration challenge is elsewhere: the state machine.
A document has a lifecycle
Draft, sent, awaiting signature, signed, expired, cancelled. Each transition must be reflected in your database, triggered by the provider, not guessed by the interface.
As with payments, webhooks are authoritative. You secure them, make them idempotent, and handle the case where they arrive out of order or twice.
Orchestrate around the signature
A signature often triggers a sequence: PDF generation, confirmation email, access activation. This orchestration must be resumable if a step fails, without resending the document for signing.
The signature is just one event in a flow. It is the flow you design.