Hi,
Access levels use joomla user groups to restrict access to products or categories.
Suppose that you have a product A and that you only allow it for the user group B.
Then, all the users registered in the user group B will see product A and others won't.
It can also be configured per category, etc.
That can be done with the Business edition of HikaShop.
HikaSerial, another extension that we sell on this website, has a "consume" mechanism where a registered user can enter a code and then, based on the configuration of the "Group consumer" plugin of HikaSerial, it will add the user automatically to the user group defined for that code. Codes (or serials as they are called) can be managed in HikaSerial.
Thus, by combining both systems, you could have a system like you want.