If a customer is having trouble with the link for a TpT Product, first check the Google Broken Links doc.
If the link is in that doc, it is likely that it has been fixed already. If that is the case, you can send the customer the canned response titled "Share Request - TpT Product" (and fill it in appropriately).
If the link is not in that doc, please notify @products so it can be fixed. Do not send the canned response. Simply send the customer a link that will work for them. You can also add this file to the Google Broken Links doc.