To create a promo code, go to Site Settings > Payment systems > Promo codes.
Enter a discount code. It should be a combination of letters and numbers, up to 12 characters in total. Set a discount type: percentage off or a flat amount off in numbers (the currency of your promo code will match the currency you use in your shop), set the number of promotional codes you'd like to offer (if the restriction is not necessary, leave the field empty or set it to 0), finally, set an expiration date.
Go to the page with the cart, select Content and add the Promo code field. It will appear in the order form.
The discount code applies only to the total price of all goods. It does not apply to delivery charges or any extras.
The promotional code is not case sensitive – if you set the code as HELLO (all caps), hello (lower case) will work just as well.