This may not work for everyone but it worked for a friend of mine tonight. The buyer wanted three items but there was no link to request a total when she used the cart and it said the seller doesn't offer combined postage.
When my friend ticked the box under Selling Preferences (shown below) to allow combined payments for up to 3 days, the buyer then got the link to request a total and my friend was able to send an invoice with the correct postage. It may just have been that the glitch disappeared, but maybe changing the settings did the trick.
After you click on the Edit button that I've circled, you get the screen below. You can have it set from 3 to 30 days but unless you do auctions that end at different times I think 3 days is plenty of time.
