How is the amount of a subscription set?

By the offer. The merchant sends the customer to checkout with an offer ID, and the amount, currency, rhythm, first payment and end rule are read from that offer. The amount cannot be edited by hand; changing it creates a new offer version that applies from the next charge.

The offer holds the terms

A subscription has no amount of its own. It points to an offer, and the offer defines the amount, the currency, the billing rhythm, the first payment, the trial if any, and the rule that ends it. At each billing date, the platform reads the offer and charges accordingly. The merchant sends the customer to checkout with an offer ID; the customer accepts the offer, and the subscription is created.

Changing the amount

• Editing the offer creates a new version, applied from the next charge to every subscription on it.
• Moving one subscription to another offer changes its terms at the next billing cycle, without proration.
• Adding a one-off charge adds an extra amount to the same subscription's order history.

Why no manual edit

An amount typed by hand on one subscription breaks the link between what the customer accepted and what is charged. Versioned offers keep that link: for any charge, the platform can show which offer version the customer agreed to and when. That is what consumer rules on subscriptions require, and what support teams need when a customer questions a charge.