I am not sure i understand correctly since i dont have a seller account to see the changes. I guess with the new changes the invoice stage no longer exists and buyer can buy and pay instantly. Seller cant refund and cancel the order if postage turns out to be more expensive ? In this case Discogs still claims its fee ? It's a valid reason and only assholes would leave negative feedback in this case, and no serious potential buyer would worry from it anyway.
From buyer's perspective, mine, it's good to know shipping costs / policies beforehand. Either that be written in seller's terms or automaticaly. To avoid surprises.
Maybe Discogs doing this cause they want to cut communications between buyer / seller ? I, many times, when i ask for shipping costs, i offer to do it out of Discogs, so i, he, we save the fee.