How do I switch 3PLs (or add one) on Shopify Plus without overselling during cutover?

Do not publish the new building until goods received complete. Do not leave the old one on the channel map once you cut. Cutover is inventory, open orders, and (if you have retail EDI) a retest. It is not a Shopify location rename. The failure mode is double-publish: Shopify selling the same units from A, from B, and from a truck that is in neither.

Freeze transfers and move on transfer orders

Freeze transfers the week you move. Raise a transfer off the old 3PL against a transfer order, not a dummy sales order. Units sit in-transit, valued, in neither building. The new 3PL receipts them. Only then does new-site on-hand go up. Sellable is not "the truck left." Sellable is GRC into the primary storage zone. Staging, pending putaway, and their receiving dock stay off the channel. Their on-hand is a file, not a live pick face. Their receiving report can show dock scans that are not yet in a pickable bin. If you export the new location the morning the trailer arrives, Shopify will sell units that are still in shrink-wrap. Publish A and B for those same units and you sell them twice. Pending is not sellable. Adding a second 3PL has the same rule: B does not publish SKUs that A still publishes, or that are still on the truck between them.

Dual-run the orders during cutover

Dual-run the orders. Old 3PL finishes what it already holds. New 3PL takes new orders only after the first cycle count agrees with your expected qty, and only after you remove A from the channel map the same hour you add B. Turn off continue-selling-when-out-of-stock. Hold a buffer sized to the lag: some 3PLs only send a count file, not a live feed. Do not babysit Shopify quantities by hand. The next export overwrites the type-in, and the typed-in number was already a lie. Map B as its own Shopify location. Do not rename A's location and hope the new building inherits the count.

Retest EDI before going live

If Target, Costco, or Nordstrom already send 850s, this is a retest, not a config swap. Sandbox PO from the VAN, confirm the 855, warehouse shipping order to the new 3PL, shipping advice back with a real carrier and SCAC, shipment marked done, outbound 856 and 810 checked, carton labels recertified against the routing guide. Confirm the new vendor code, LTL carrier, and SSCC prefix before the first live 850. Then change the 3PL on the trading partner. A missing SCAC or a test ISA flagged as production is a chargeback on day one. Reconstruct by SKU and timestamp before you call the first week an oversell.