One-time Purchase Code - can't checkout or add to cart without it?

Biopharmaxie

We want to limit the purchase of these supplements to those customers that have a code (or username & password) from a nutricionist. Looking for a one-time code to be able to checkout.

For their next purchase they would need a new code.

Perhaps Discount Code module. Making it REQUIRED on the shopping cart and hiding Checkout button until you enter and apply a valid Discount Code.

thanks.