Seller sent wrong invoice on eBay?

I purchased a few items from a seller on eBay. I was told that they would combine shipping for me.

When they sent me the amount, it was $20 postage, this combined amount was also stated under "my purchases" history.

However, this amount did not show up when I tried to pay through the eBay checkout - each item still had their additional postage costs.

I then sent a message back to the seller, asking if they could please send me an invoice so I could pay the new combined amount.

They then sent me an invoice that had almost double the amount for combined postage - instead of the $20 it was now almost $40! They also sent me a message apologizing because the amount on the combined invoice was now wrong but eBay would not allow them to change it.

They tell me the correct amount to pay (with $20 postage not the $40) but how can I do this if the amount on the invoice is wrong?