Is the seller refusing or baffled?
If the former, you can Report him as an unresponsive seller, but if he is baffled he can send you an invoice for your entire purchase, including combined and discounted postage through Paypal's Request Money/invoicing service.
You and he are covered by PP's customer protection policies. So no worries there.
He should give the eBay transaction numbers in the text box of the invoice.
He will have to mark the items paid and shipped manually, since PP can't do this automatically when he sends a PP rather than an eBay invoice.
BTW, one reason that sellers may have difficulty with invoicing multiple purchases is that he may have listed the items on various sites. This is not uncommon with Canadian sellers. I list some of my items on eBaydotCA (Canada) and some on eBaydotCOM (USA). They look identical to buyers, but these sales cannot be combined automatically.