What do you do if a small vendor doesn't keep their promises?

[cessna152towser]

I would agree with CMcG here.
I've also met Peter (McHummel) and he is a decent guy, if somewhat laid back.
If I were you I'd try to push this a bit further to see if you can reach some sort of compromise with him and then report the outcome back here.
Try to find a win-win situation.
For you to lose funds and for Peter's name to be tarnished would be a lose-lose situation for you both and would please nobody.
I once had a problem with a part-time kilt maker who took my money up front via Paypal then failed to deliver the goods as she had family issues. However I persevered and she eventually delivered an excellent kilt after more than a year's wait and we are still friends.