After you agree to specs, here's a better way to proceed:
50% down non-refundable at time of order (filters out the guys who only want to kick the tires, and covers your inital costs in wood & hardware if customer decides to back out)
50% upon completion, due in 30 days or you retain ownership of the instrument (puts a deadline on the final payment so you don't get strung along, and if he stiffs you then you still have something to sell)
Make shipping part of the final cost
X-year warranty against workmanship, during which you fix for free (you split the shipping)
All of the above + build specs are put down in writing and customer signs it - this becomes the work order and a binding contract (avoids the "that's not what I told you" arguments down the road, and customer feels like you're bound in writing to build his instrument)