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Your legal position is that you have a contract, partly verbal and partly contained in writing, and that your client is in breach of contract for failing to pay you in accordance with that contract. You have fulfilled your side of the bargain. From your narrative you have worked above and beyond in order to get the product to the client. Your remedy is to issue a written letter requesting payment in accordance with the contract. If this is refused or ignored you can sue them in the sheriff court. Depending on the amount involved will depend on which procedure you can use, i.e., small claim, summary cause or ordinary action. Happy to discuss further. In the meantime I hope that helps. Please leave a positive rating so that I am credited for my time.