PayPal's service (the standard web forms, which your service is competing with) has them all over the place, such as https://www.paypal.com/cgi-bin/webscr?cmd=x

PayPal's service (the standard web forms, which your service is competing with) has them all over the place, such as https://www.paypal.com/cgi-bin/webscr?cmd=xpt/Marketing/demo/wpstandard/WPStandardDemo3-outside https://www.paypal.com/cgi-bin/webscr?cmd=xpt/Marketing/demo... It is generally expected that you have a cancel option (to cancel the transaction). Every online system I've used for payments has a clear "cancel transaction" button.