Coins are credits that, unlike booking credits, have actual monetary value. They correspond to a specific amount of money, with one coin equaling one unit of the chosen currency. You can add coins through a billing plan, or in the customer's profile. A billing plan lets you define the "valid for" period of coins - monthly or one-off.
2. Add the amount of coins the customer should get either in the billing plan (if the amount will be the same for all customers purchasing this service), or directly in the customer profile (if the coins amount is variable).
Further Reading - Printing coins - how to use them in OfficeRnD?