With the latest Soapbox release, we've added support for promo codes in the Shop app. You can now create promotional codes that offer visitors discounts off the list price of one or multiple products in a given store. Define the code, limit the number of times it can be used, set the start and end date of the promotion - and then share the code with members, students, supporters, or whomever you'd like to offer reduced prices for the subscriptions or swag you're selling with Shop.
For more details, check out the following:
Creating promo codes for products in a store
Embedding promo codes in URLs for stores for ease of use
Happy Soapboxing!