We add a new field to your customers: "Member yes/no".
The customer can switch membership on and off via a button under "My account".
As a result, the customer will automatically receive a credit of y percent x days after his order (not possible for guests).
You can set the values x and y yourself in the admin, under Modules - Class extensions - product
For ALL customers we show under the product how much they would save as a member.
For the module we need to set up a cronjob in your hosting account. If your hoster does not allow cronjobs, we can set one up for you.