Build-a-Box: fix for discounts silently dropping to zero at checkout

A reliability bug in Build-a-Box has been fixed. Behind the scenes, Subi registers which products are eligible for a Build-a-Box discount by writing an eligible_box_ids metafield to each product in Shopify. If that write failed for any reason, such as a transient Shopify API error, the product silently lost its discount eligibility at checkout, with no error raised anywhere.

The write path had no retry logic, even though the equivalent delete path already did. A single transient failure was enough to drop a product's eligibility registration, and because the failure was swallowed rather than surfaced, nothing flagged it for the merchant or for Subi's support team. The discount would simply stop applying at checkout until someone happened to notice and re-sync manually.

The fix brings the write path in line with the delete path: it now retries on transient errors, and if a write still fails after retries, the sync failure is surfaced as an error instead of being logged and silently treated as a success.

If you run Build-a-Box discounts, this means fewer silent gaps between the products you've configured as eligible and what customers actually see discounted at checkout, and if a sync issue does happen, it now surfaces instead of going unnoticed.