Create and update Sync Variants under a Printful Sync Product (v1 Products API)

Steps

1. Authenticate with your Printful API token (OAuth) against the v1 Products API — Sync Product/Sync Variant management is not yet available in v2.
2. When creating a Sync Product, include a sync_variants array; each Sync Variant must reference a Printful Catalog variant_id and one or more print files.
3. Attach print files either by an existing File ID from your store's file library, or by URL (files with a matching URL are reused rather than re-uploaded).
4. Optionally set an external_id on the Sync Product/Sync Variant so you can reference it later using an @-prefixed ID instead of Printful's internal numeric ID.
5. To modify an existing Sync Product's variants, resend the full sync_variants array with the IDs of every variant you want to keep — any existing variant you omit is deleted.