In my experience, charities try to get you on a recurrent donation nowadays instead of taking cash or transfers (although I am in the Netherlands, not Belgium). It’s terribly annoying because they take the “being lazy and forget about it” and weaponise it against you.
Oh don’t worry. If you try to deposit it at a bank, they’ll start asking questions right away on how you got the money. Unless you never bring it into the “official” system, the financial surveillance system will find it.