I just bought two items from the same seller on two separate days(actually only 2 hours apart, but on different days iykwim).

In order for the seller to adjust their postage to represent a combined postage total, I had to send a request for an invoice (can't remember the exact words) so that the seller could adjust the postage amount and combine on the one invoice.

Same thing happened with another purchase, after I won, I asked the seller if they still had something from a few weeks earlier they had listed, so it took an hour or so for them to relist and me to buy, but I had to go through that same process.

One was an aussie seller, the other an international transaction.

I've never noticed this before, either as a buyer or a seller - as a seller, the items have always magically correlated and I could just change the postage price at the bottom of the invoice before sending it.