No need to panic! The risk is when scammers ask or your email, and then send a convincing fake email "from PayPal" pretending that you have been paid, and telling you to post the item.
Nothing like this is likely to happen in the present situation.
By the way, if your buyer as already paid the full amount stated in the listing I don't think this arrangement to upgrade the postage will count as a split payment. The sum it sold for is all that eBay will be concerned with.
Strictly speaking you've unwittingly managed to avoid paying eBay's 10% fee on the extra postage cost, but I think they will survive!
