Linked products?

What’s the best way to make products linked so that one product cannot be purchased during checkout unless another is also in the cart? For my use case, it wouldn’t matter which is added first, just that both need to be in the cart when checking out. Also, is it possible to make it more broad…as in so that the required product just needs to be one from a specific category?

To explain my use. I have a specific design fee setup for a custom business card design. I’d like to make it so that the fee cannot be purchased at checkout unless a business card product from the “custom” category is also in the cart.

I have it so that the “fee” product is hidden and the only way to add it to the cart is if you’re on the single product page for one of the products in that “custom” category. I’d just like a failsafe so that people cannot get the discounted design fee by itself.

Any creative ideas?

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