"Jim,
Be careful doing business like this! You might find that MANY of your unknown patrons “conveniently” have a problem with “something” concerning your work and you end up with an epidemic of working for free! "

I agree. There is nothing wrong with receiving payment with the part.
Your reputation will work for you and you are the type of guy that will always work something out if a problem arises.
You can always send a refund.
Jules