In advance or in arrears
Charge at the start of a period or at the end.
Every price bills at one of two moments. In advance means the customer pays at the start of the period, before they use it. In arrears means they pay at the end, after. This is set on the price.
In advance (prepaid)
The default. The customer is charged when the period begins. A monthly plan billed in advance charges on day one for the month ahead. This is how most flat subscriptions work: pay first, then use the service.
In arrears (postpaid)
The customer is charged when the period ends, for what the period covered. This fits anything you can only price after the fact.
Important
Usage is always billed in arrears. You cannot price consumption before it happens, so metered and usage-based prices are billed at the end of the period no matter what, and the setting is forced to arrears for them.
A per unit price for something the customer consumes gets totalled up and billed when the period closes.
Mixing both on one subscription
A subscription can hold items with different timing. A plan billed in advance can sit next to a usage add-on billed in arrears. Each item follows its own price's timing, so the customer prepays the base and pays for usage after it is measured.
Edge cases
- Timing lives on the price, not the subscription. Two prices for the same product can bill at different moments.
- The default is in advance. A price with nothing set bills at the start of the period.
- You cannot force usage into advance. The engine bills metered items in arrears regardless of the flag, because the amount does not exist until the usage does.