In the system, you are able to give your customers In Credit funds to apply to your services. This can be used in a number of ways. For example, if a customer wants a refund but you aren't going to refund it in cash then you can use this feature to give them that money to use in the gym for services.

To add them to a customer's account, head to Members > Search for the Member > Click into Profile.

Once in the profile, head to Finances > Payments > New Payment > Set as Unassigned Payment. Fill out the form with how much you want to give the customer to use towards a purchase and note what it is and what it was given for your own record. Once you save, it will credit that In Credit that you can apply to an invoice.

If you want your customer to be able to choose that as their payment method, you will need to enable the setting under Settings > Member Area > Classes. Click the drop-down next to When members try sign up to a class online: and chose "Force members to pay online (or use their existing 'In Credit' funs)...". These settings only affect only bookings (classes/appointments).