[bugrasan]

I'm not aware of any online shops where you can "tip" on top of the payment - e.g. for postal service.

Also, a "tip" is defined (at least for me) as "a thank you for a service well provided". Therefore I can't calculate in advance how satisfied I will be with the delivery, e.g. if it's on time and the person is friendly then the tip will be larger, versus late and cold pizza...