How do I transfer inventory between two US warehouses (or warehouse → store) on Shopify without overselling what's on the truck?

This is a stock move on one set of books. Same US entity, two buildings, or warehouse to store. Transfer order, reserve at origin, in-transit, receipt. There is no customer and no transfer-price invoice. If you skip a step, one site is missing units and the other is selling air.

Not an intercompany shipment

Do not treat this like an intercompany shipment to a second company. You are not raising a bill. Cost stays on the same layer. The units keep the landed WAC they already had. A dummy sales order from Warehouse A to Warehouse B will look like revenue and shrink. A pair of inventory adjustments will lose the truck. If a second legal entity owns the destination, that is a different post (transfer-price invoice, two books). This one is same-entity only.

Reserve, ship, and track in-transit

Raise a transfer order for the SKUs and qty. Reserve those units at origin so the origin storefront cannot sell what is about to leave. Pick and ship against the transfer, not against a customer order. The units leave origin on-hand and sit in-transit: still yours, in neither building, not available-to-sell. Do not publish destination Incoming, Incoming Transfer, or any inbound bucket as sellable. Shopify will take orders against a truck that has not docked.

Receive partials at destination

Receive at destination, including partials. Forty of sixty can land today. Twenty stay in-transit until they do. Selling the forty as sixty is how the store oversells the rest of the truck. Warehouse to store uses the same pattern. The store is a location, not a customer. Do not publish the store's incoming transfer as walk-in or ship-from-store stock. Ready-to-ship, In transit, and Transferred are states on the transfer, not a reason to add inbound to available-to-sell.

Month end, returns, and 3PL receipts

In-transit at month end is still inventory, just not in a pick face. Cycle-count neither building for those units until receipt. Returns of a bad transfer go back on a return transfer, not a Slack adjustment. If a 3PL is either end, they must receive the transfer order, not a new ASN you invented. Shopify will not auto-create replenishment transfers. You raise them, or the East warehouse sells out while West sits fat. A 3PL that receipts a transfer as a purchase order will invent a second cost layer and a fake AP bill. Kill that document. Receive the transfer itself.

Same entity. One cost. Reserve, move, receive. The truck is not a store. If destination ATS rose before the receipt posted, someone published Incoming. Pull that publish down before the next wave, then receive what actually docked.