Coins are a type of credits, which unlike booking credits have actual monetary value, i.e they correspond to a specific amount of money - one coin is equal to one unit of currency. You can either add coins through a billing plan (which defines the "valid for" period of the coins - monthly vs one-off), or in the customer's profile.
1. Configure your resource rates and bookable resources.
2. Add the amount of coins the customer should get either in the billing plan (if the amount would be the same for all customers purchasing this service), or directly in the profile of the client (if the coins amount depends on various things).
Additional resources: Printing coins - how to use them in OfficeRnD?