Don't offer a service if you can't fulfill it. Doesn't matter that it was free really, they still messed up. People's time is important.

Despite all that, everyone makes mistakes, it is how you react to them imo. Missed a good opportunity here and burnt bridges all round. Could have just have easy got in touch to say "Really sorry about that Mossy, this free offer has us very busy, why don't you call in tomorrow and we will get it sorted" then give a voucher for 20% off next parts order and a free valet or something.

Chance to keep them as a customer and spin the whole thing around. Might have even had him recommending them despite the initial issues. No chance now and you can be sure he will be telling others to avoid on his personal experience.