IDEA: Avoid arguments with vendors. Complete a detailed P.O. before work starts to avoid problems/billing ‘surprises’ later.

Tell your customer to issue you a written purchase order defining quantity, price, milestones, return policy, penalties tied to performance, payment terms and any buy back agreements of excess material and supplies. If the company does not issue purchase orders, you as the vendor can write a sales acknowledgement that states all of these criteria for the customer to accept and sign. If he refuses to sign anything and wants nothing in writing, rethink accepting this customer. Courts will ask for written agreements over certain dollar amounts(verbal agreements) in every state (text messages, emails, contracts, voicemails possibly).  If nothing else, write down the specifics of the agreement (price, quantity, who pays for freight and how much, taxes, other shipping fees or gas adjustments, etc. and send them to the buyer to accept. Once he writes his acceptance (I agree or I accept your proposal) to that same email, you have your agreement in writing, not the most professional but enough to win in court if necessary.