Within PowerPanel you can give your customers the option to have credit in their account which they can use to pay for services.
To start offering this service to your customers you will have to enable the prepaid option first. Go to System -> Settings -> Invoice settings and scroll down a bit to look for the prepaid settings.
After you have enabled the module the rest of the settings that are available for the prepaid module will appear:
Here you can set the minimal amount that your customers have to pay to upgrade their funds in their account. You can also set the description line for the invoice that will be generated, if you set this line the default line (which is multilingual) will be overwritten. The last option is the product group on which you want to have it booked in your accounting software.
If you have any questions feel free to get in touch.