Product variants setup

My vendors are increasingly requesting a feature that allows them to add multiple items in a single listing, similar to how product variants work. Since my site focuses on sports cards, they would like to list one card to represent a set and then include each individual card (along with its quantity) for customers to choose from.

I’ve tried using variants, but it seems I can only add one item to the cart at a time. Ideally, I would like customers to be able to select one or more cards from the list before checking out.

Could it be that I haven’t set this up correctly? Any suggestions?