First, go to tools -> options from the invoice screen.
Go to Invoice Settings
Uncheck Combine Lines, and set Promotions to Automatic.
Hit update.
Then you will need to set up promotions you want to use. Under view -> promotions, add promotions you want (see Promotions note for details.)
Go to inventory management, then create a general discount card item. Check the "This is a discount card" box, then hit the "promotions" button to set the discount card promotions that will apply for users of this card.
You will now be able to sell this item to individual customers for their unique discount cards