If you have promotions running, they can't.

If they are using the app, they can't.

Get them to give you a list of what they want and set up a specific listing for them at the negotiated price and postage.

After a series of emails with a buyer about the non appearance of the "request total" option, we worked out that they had not hit the "commit to buy" button.  Which makes sense in that they wanted to know the  postage total first, but ebay doesn't take this into consideration.  I ended up combining their list of items into one.

Thanks everyone!

I appreciate the advice, and we got there in the end with the buyer.

Someone mentioned that making the listings 'Require immediate payment' also has an impact and stops the "Request total" from working. Has anyone else found that to be the case?