If you have a dispute with a customer regarding the applicability of your general terms and conditions of sale, you must be able to show that you have made them repugnant to third parties. In other words, you must be able to demonstrate that the customer has had the opportunity to read and accept the general terms and conditions of sale.
Below you will find 2 ways that you can combine to do this:
By adding a note to your documents in the nature of: "By approving this document, you accept our general terms and conditions of sale attached and available on our website".
By putting your terms and conditions of sale as an attachment or directly into the document as described here.