well, I don't know for sure since I haven't done that..
however, you could have 'products' that are a 'donation'.. and have them at pre-set price levels.
Maybe come up with a graphic for the 'donation products' that shows a person helping someone,, just a thought..
edit:
in the pro version, you can have different prices per 'size' of item,,
Use that and have it normal price, normal price plus $xx donation, normal price plus $zz donation and so on...
Yeah, I guess I'll have to go that route. Not as elegant, but workable.