I have a brick and mortar store and I want to keep all of the items purchased online at our physical store until the registry is complete. How can I avoid customers having to pay for shipping?

Create a 'Registry' shipping option on the backend  of Shopify for free. You can also add a pop-up message under Preferences > Emails & Notifications > Pop-Up Notification. Customers will see this message the first time they shop from a registry.