Many auto repair shops are adding a fee to the final invoice for customers using credit cards. I get it, but don't agree.
For me it's simple. First, do your best to negotiate the best deal from your credit card provider service. Next, take that fee and add it to your cost of doing business. To me, I consider this fee an expenses, just like all other expenses: office supplies, utility bill, insurance, taxes, training, travel expenses, maintenance, etc. etc.
From your total average monthly expenses, you will be able to determine your breakeven, and from there, set your net profit goal. In other words, forget about the charging the customer a fee, just build into your overall prices. You will accomplish the same thing, and not bring attention to the customer that small fee that may be a big deal.