Put the customer on one of the routine variants their scan produced. The persisted routine (data on GET /skin/care-routine/{batch_id}, the tracker, the shelf, the vendor views and the result email) is rewritten from the chosen variant's snapshot, and selected_variant flips. Idempotent: selecting the current variant returns the same rows. A change of variant is tracked as a Routine Variant Selected analytics event carrying source; nothing is billed or emailed by this call. Widget (Bearer) and mobile (cookie) callers use it alike; a vendor caller must own the scan. A referred face or body scan returns 409. Budget warnings do not block tier selection. Essential uses one face-and-body budget with the existing 10% allowance; price and concern coverage remain visible when the remaining core exceeds it. Referred sibling zones are excluded from the selected routine.

The tier is one choice per scan. Selecting on a face batch (or on a body-only session's first zone) rewrites every body zone hanging off it to the same variant; selecting on a body zone rewrites the face and its sibling zones. Members with no variants of their own (a routine from before the feature, a chat-rewritten face) are left as they are. family carries the other members' new rows so a client updates every zone from this one response.