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# Subscriptions and One-Time Purchases Billing

Commerce Engine is a single platform that provides support for both subscriptions and one-time purchases without compromising functionality for either type of transaction. A one-time purchase is exactly what it sounds like: a single transaction.

Subscriptions and one-time purchases share many of the same components such as a product, a price, and a payment method. Subscriptions require the system to have pre-defined products as part of the product catalog. One-time purchases don't require this. Rather, one-time purchases allow the system to create the product on the fly if it was not previously defined.

Recurring purchases are stored as subscriptions in the Commerce Engine system. The subscription element in the system (per customer account) generates a transaction every time a billing action is required. One-time purchases are recorded under a customer's account and are a single transaction.

In today's markets, it is common to offer both subscriptions and one-time purchases to the same audiences. For instance, a company that offers runners online coaching subscriptions could expand its offering to include one-time purchases of athletic clothing and accessories for the same market.

Commerce Engine supports one-time purchases with the same broad functionality provided for subscriptions, including discounts, sales tax calculation, entitlements, and security. The only difference being that this happens once and no recurring billing is required. Retry functionality is not enabled for one-time purchases.

## Transactions in Commerce Engine

In Commerce Engine, transactions can be one-time purchases or recurring purchases that are automatically generated by a Commerce Engine subscription. Commerce Engine transactions typically represent requests for funds sent to a payment provider and they encapsulate data about the financial transaction processed through Commerce Engine.

## Real-Time Billing

Commerce Engine uses real-time billing whenever the system needs to calculate and authorize a charge. Accordingly, real-time billing occurs for new or modified subscriptions. One-time purchases are also processed through real-time billing.

**Parent topic:**[Charging and Billing](chargingBilling)


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