Unfortunately I don't think Paypal can help you with that. If the seller isn't responding, I'd actually phone eBay customer service (unless they have a live online chat function now? I don't know) and ask if it's a problem they can resolve. But it sounds like in this case, the seller just hasn't sent you a proper invoice yet with the discount applied. Is there any notice on the seller's store page about being on vacation or to expect a delay for some other reason? I'm pretty sure they're supposed to send you an invoice and respond to messages within a certain time, otherwise.

It's been a while since I used eBay as well because the whole payment system seems to be broken.