Why did I get API error code 10621?

PayPal returned API error code 10621 - Order has expired.

The order you're trying to authorize, capture, or void has already expired. Orders remain open for 30 days, and after that period you can't capture funds for the transaction.

If you use the GetTransactionDetails API on your PayPal Order ID before performing an authorization, capture, or void, the value of the status variable can tell you whether the order has expired. If it has, take no further action with the order; close it out of your system.

See also:
PayPal NVP PHP code examples