If people see that you're selling merch, they'll think "They're making a fortune in merch, I don't need to donate," and then they won't donate AND they won't buy the merch!
Just do a donation meter. When it gets full, everybody gets a little something and the ones who donate get a little something extra. Simple.
Remind people that there are no ads (most won't notice, or assume there are ads they've blocked) and ask politely for some money. Be straightforward about it. Don't be sneaky.
