If a buyer posts a positive rating for a solo ad, he'll see an option to add a tip for the seller.
If the tip is sent, the seller will receive a notification in the Messages section and the transaction will be reflected in the Money section. A tip can be given only once per a solo ad.
A buyer can select from 1% to 15% tip. The buyer can cover the commission for the transaction. If the commission for the tip is higher than the tip itself, the option will be greyed out as 1% box on the screenshot.