You've received platform credits as per the Heroku for GitHub Students program. However, you've received an invoice for monthly usage and are curious as to why you're being charged.
Summary of maximum platform credits available per month
When you are granted platform credits from the Heroku for GitHub Students program, you receive a total allocation of $156.00 USD, which is applied in increments of $13 per month over a 12 month period. This means that the maximum amount of credits that you may use in any given month is a total of $13.00 USD. Unused credits will expire at the end of the month and will not roll over to the following month.
- If you receive an invoice for $13.00 for June and have the Eco Dynos plan ($5 for 1000 dyno hours/month), one Mini Heroku Postgres database ($5/month), and one Mini Heroku Data for Redis database ($3/month) running, you'll receive an invoice for $13.00 and the balance due to Heroku will be $0.00.
- If you receive an invoice for $50.00 for June and have higher-cost Heroku products running (such as a Standard 2X Heroku Dyno, or a Standard Heroku Postgres database), you'll receive a credit of $13.00 on your invoice and the remaining balance due will be $37.00.
For more information on how platform credits associated with the Heroku for GitHub Students program are applied, please reference this help article.