Starting today itch.io now lets you apply a minimum price to individual files. For example you could offer your game at 5 dollars, and then if someone pays over 7 dollars they also get the game soundtrack! Another possibility is having your game be free, but any extra downloads have a minimum price.
You can set file prices in the game edit page next to each file you have uploaded:
When someone goes to buy your game they get an updated interface that lets them see what extras they’ll currently get, and if they click on something not unlocked it will raise the payment price: