Payment invoice

A crypto payment invoice is a request for payment that specifies an amount, asset and receiving address and is tracked on-chain until it is fulfilled.

An invoice turns 'you owe X' into a concrete on-chain expectation: a specific amount of a given asset at a given address, with an identifier and expiry. A checkout presents it to the payer, and the processor watches the chain for a matching transfer. When one confirms, the invoice is marked paid and a settlement webhook fires. Our payments primitives create invoices, render checkout, and emit HMAC-SHA256-signed webhooks so a merchant's backend learns the moment a payment confirms.