Everyone does business differently. If it was a small amount, I usually would just cover it myself; or in a case that I delayed the work, I'd cover the Priority Mail myself, but not every seller the same...
In this case, if both the seller and buyer agree on paying for extra shipping, that's fine too. I just thought the seller should be more proactive on communication. From my point of view, if you as a buyer is willing to pay for extra shipping, I'd just set up a listing for you to pay for that amount. It is as simple as that and I don't get why it gets so complicated... be honest... :)