I'm having some issues understanding the price of the Mistified bundle.

It's 6.15 euros on the main page, but appears to be 7.55 euros when I click the link
http://www.gog.com/promo/big_fall_myst_bundle_201114

Why?

Read again. It's not 7.55 euros. It's 7.55 dollars.

Oh.

So, can I decide to pay in dollars instead of euros?

Yes, go to the bottom right part of the main page and you will see an option to change your currency from euros to dollars, if you so wish. The price, though remains the same; 7.55 dollars which, when converted, is approximately 6.15 euros.

At the bottom of each (store) page - ie. game/movie info pages and catalogue, as well as front page there is a menu to switch between US Dollars and local currency.