Two scriptures came to me - one is "let the left hand not know what the right hand is doing" - ie giving a set amount you think about too much isn't the point.

The other is the story of the little old woman who gave a penny to the collection after the rich men had poured in loads of money. She gave the most, explained Jesus, because she gave all she could. The rich men weren't bothered about the (to them) tiny amount they gave. It was just for show. And that is there reward on earth.

I tended to give via direct debit (when I was a regular attendee; the church had a little plate at the back that wasn't passed around and I'd give more if they were collecting for a special thing I wanted to help with): if I don't have a lot of money I'll raid DH's wallet. I'll give as much as I can at the time.