回複：現他威脅要放lien到我的房子上。我該如何應對?

Document everything. Try to negotiate with him first. Show him the quote. Get a proof that he agreed to build the door with mail slot for you, better a written one. If he wants to put a lien on your house, so be it. You can always counter claim willful exaggeration with malicous intent. The only thing you need to consider is whether it is worth the $100 for all of these.