When you are using the Point of Sale to sell products, you might choose to sell any one-time payment product. For example, you can sell day passes or 10-time passes through it. When you are using credits in your system as well, you can add those to the products.
When you're creating a product, click on "advanced options," and you will see the option to choose a service to attach credits to. Here you can attach a certain amount of credits to the product. You can also set up an expiration date for the credits, but this is optional.
When you sell the product through the point of sale, it is mandatory to attach the client to the product, because the credits will be automatically added to their account once you've sold the product. Please note that the invoice does not have to be paid directly for the credits to become accessible, so if you want to use the invoice in your next SEPA batch, there is no hurry.