Hello,

One option is described here - devnet.kentico.com/Knowledge-Base/E-commerce/Direct-PayPal-payment.aspx.

Second option is using BizForms - you could e.g. create simple BizForm with one field for amount of donation and in OnAfterSave event handler of BizForm (http://www.kentico.com/docs/devguide/modifying_the_code_of_standard_web_parts.htm) put custom code to ensure redirection and passing of the information to on-line payment gateway.

Best regards
Ondrej Vasil