Let's face it, the reason people say donations is because they want you to pay as much as possible
[You must be logged in to view images. Log in or Register.] if they say porting for 10p people will pay 10p, where as someone else might pay 50p if you don't specify.
It's very similar to tipping waiters in restaurants, etiquette seems to say that 10% or w/e is the correct amount.
Refusing to do something when you're claiming you're doing it for donations is a pretty poor show though.