The only way to do this is through "DLC," creating a project for each deck you want to sell and activating the condition that they must already own the base project to purchase that DLC.
The downside is that Itch doesn't have a shopping cart, and if someone wants to buy your eight decks, they'll have to make eight separate purchases, resulting in eight transaction fees.
Itch doesn't allow you to selectively purchase files within the same project. Instead, it has a Tier system, designed so that people "donate" a higher amount than the suggested price or the base purchase price and receive rewards based on that extra amount. Note that in the per-file pricing system, the condition for accessing that file is having made a purchase with a total value equal to or greater than the file's price.
If you have eight files priced at $3 each, to access all eight files, a single purchase of $3 or more is sufficient.
For what you're proposing, the only way I can think of is for you to have 8 separate projects/pages.