Upgrading supporter packs.

Hi, I decided to buy a DLC/supporter pack, but on Steam it includes the base game which they don't exclude it and give you a discount... it's 26.99$ for where I live.

When I went to see if I can upgrade to the same pack on the in game shop I saw that it would costs me 30$ to upgrade...

When I asked someone about this on the Discord server, they said it costed them 15$ to upgrade, while for me it is 30$.

Is there an issue? or did they change the prices.