I'm glad you asked, because now I can explain it for the funnys. Basically,
Buy Undertale - Game + Soundtrack Bundle
reads like "Product minus game plus soundtrack", which sounds to me like basically just the soundtrack. The person wants to use an em-dash---like this one---but instead they used a hyphen (-), which when spaced like it is on the Steam page is naturally read as a minus sign, especially when there is already a plus sign in the same expression.
But this also costs a lot more. So you can pay $9.99 for product, or $17.99 for the product without the game, but you get a soundtrack.