How to separate the checkout process for buying credits?

For when people don't have any credits and it's in credits only mode:

When a person tries to buy a membership, the membership gets added to the cart, but don't have any credits yet. It tells them to buy credits after they click on the checkout button. So they click on buy credits and then that gets added to the cart too, but they can't buy them because other things are in the cart.

It's like "What do I do, How do I buy the membership?"

If it's in credits only mode and the person doesn't have any credits, then I think that buying credits should be a direct checkout without sending it to the cart. Disable the option to buy anything from the cart unless they have credits. Then put something like a "Buy Credits First" button in place of the "Checkout with Credits" button and open the credits buying page.