In more formal English, we'd probably say, "May I bring them to donate?" Less formally, though, "can" and "could" are pretty much interchangeable. Generally, we use "could" (and other subjunctives, like "would" and "might") to make our requests a little less abrupt; "Can I see that?" is a bit more brusque, I think, than "Could I see that?"
'could' is more deferential, and in my opinion, it's not necessary to be deferential when making an offer. You're not asking for a favor; you're offering to do a favor.