I asked an acquaintance from my church if he would sing during my wedding ceremony, and when he said he'd do me the honor he never mentioned being paid. Now I don't want to insult him by offering to pay, but I do want to acknowledge my gratitude. Would a gift certificate to a nice restaurant along with a thank-you note be acceptable?
It seems you've already solved your problem (and without having to broach an awkward "payment" conversation). The gift you describe, along with a hand-written note, is totally acceptable. It sounds like this individual isn't expecting anything in return, and he's sure to be delighted by your thoughtfulness.
Wedding Ceremony Issues
Wedding Ceremony + Wedding Vow Ideas