How long is the delay on a item with insufficiently paid postage likely to be?

Ordered two separate items on eBay from the same seller, who shipped tracked 48 nearly a fortnight ago. So far tracking hasn't updated beyond acceptance at the post office. Seller has told me that they sent both items together as large letter with one single label and they are now wondering if has exceeded the postage limit (even though it was accepted at the post office). If this is the case will I get a card to pay the excess or will it just be returned to them? And any ideas how long it's likely to take?