I guess these are as much a question for shop owners on this topic:
To me, your door rate should be firm, but you have a quite a bit of flexibility on parts in both quality and in terms of the markup you are willing to take. When you are "negotiating" on price, do you negotiate on the price of the same parts etc., or offer other lower price/quality part options?
Also, if a customer is willing to pay cash or debit, would you consider a discount based on not having credit card fees (not suggesting no tax, just 2.5% or whatever off.)?
Looking forward to input!