If it is the seller in the UK the listing states that they combine up to 15 items for the one price...GBP3.40

The seller in the USA will combine 3 items for the price of $2.99.

Whichever one it is, try adding the postage to the invoice yourself and see if you can pay it.

Have you actually tried going through checkout again since you asked for a new invoice?  It is not uncommon for the seller to alter the invoice so it shows there but the actual invoice never arrives in your inbox.

Have you tried altering the postage on the invoice yourself?

I would not try paying to their email address or you will void your ebay buyer protection.  The payment must be linked to an ebay transaction as as soon as you put those details in paypal will automatically come up with the invoice with the incorrect postage.

Unless they open non payer disputes nothing will happen.

If they do then contact ebay and they can see your messages asking for the combined invoice as per add and they should close the case in your favor and remove any strikes if given any.

I can't ring them as they are from England.

Why can't you ring them? It is not veru expensive with most providers to call the UK and if you use Skype it is not much more than a national call.

If you want to pay and don't want them to profit from not combininf then uncombine the invoice and pay for eacj one separately preferably using friends or relatives address for two of them so they will be out a basic Paypal fee on all three instead of just one as well as having to send them in different parcels.