Using another postal service isn't against eBay policy. A seller can use whatever service they like. What you should have done was ask the seller to relist the items you bought as a single listing, then you'd only be paying one lot of postage. Some will do it. If you offer to pay an extra dollar or 2 to cover any listing fees they might be more open to doing it.
If you do message a seller about changing the postal service, you just have to be careful how you word your message because eBay currently has this drug induced paranoia thing happening where they think everyone is trying to sell off eBay, thereby doing them out of a few bucks in fees. If messages aren't worded correctly, both the buyer and the seller will be blocked from sending anyone messages for 7 days.
Something like "dear Seller. I really want to buy these items, but so I am able to combine the postage cost, would you be able to relist the items on eBay as a new single listing and I will repurchase? I am happy to pay for any extra insertion fees you may incur from relisting them".
There's a lot of people being stuck by buying multiple small items not realising you can't combine. You used to be able to get around it by requesting a total from the cart, but they closed that loophole a long time ago.
It's not always the seller's fault. Often they don't know they have been opted in and they don't know the item is being sent overseas. Sellers who normally only sell to their own country find out later their item has gone overseas when a buyer contacts them, sometimes abusing them for postage gouging. If they know they are opted in, they also usually have no idea how steep the postage cost is. If it's for a single item, you can ask them to insert the weight and dimensions of the item into the listing and that makes it a little bit closer to normal cost.
