How I fixed Shopify bundle inventory not syncing to a 3PL

I dealt with this recently in a DTC Shopify store selling bundles fulfilled by a third-party logistics warehouse. The visible problem was bundle orders looked paid in Shopify but did not reserve the component SKUs at the 3PL. The easy explanation was the 3PL API being down, but it appeared when a marketing team launched a bundle built from existing single-SKU products. That mattered because…