That's an interesting question. Ultimately, that might come down to the contract for work. If a customer has been granted the right to request certain documentation, or if that customer is paying on a time and materials basis, then it'd seem like they'd certainly have the right to inspect material receipts. But even if not - if everything is on the up-and-up, and if the customer is being billed as set out by the contract or agreement to perform work, then there should be little harm in showing a customer the receipts for materials. Still, some construction businesses might be fearful of showing their margins for a variety of reasons. If that's the case, discussing the matter with a customer and breaking down how an invoice was calculated might be helpful to work toward resolving the matter.