I don't invoice for less than a few dollars in my shop...by the time I add up the transaction fee, percentage, labor to send the invoice, and additional labor when it comes time to do my bookkeeping and taxes, it's just not worth it *for me*.
As a buyer, I wouldn't find it tacky however. If I go over, I expect to be billed for the extra - I'm already getting a bargain, after all! I wouldn't find it tacky to be billed for a dollar.