Billing abstractions that age well

A healthy billing layer gives product code a small contract: create checkout, open a portal, read the current plan, and check entitlements.

Provider SDK imports should live at the edge of the system. That makes pricing pages, settings panels, and product guards easier to test and easier to migrate.

The most useful plan configuration describes buyer-facing copy and implementation-facing identifiers together. When both live in one place, plan changes become ordinary product edits.