Hello. My problem is the following. I bought the pro pack in the store, but I really want the premium pack. Is it possible to pay the difference and have the premium pack? Can the Pro pack be returned so that I can buy the premium pack? Thank you very much and sorry for the inconvenience.
You can't return a pack, but if you own some content, its cost will be deducted from the price of a pack you buy. Basically that's how it works. But different content/packs have different discounts so it complicates things a bit.
The only case it would work like that is if you aren't logged in, or logged in under a different Steam account. Otherwise it all works automatically. Notice that Pro pack, even though it costs half the Premuim, doesn't have half of the game content, so your new price for Premuim pack won't be 50Euro, more like 75-80 Euro.