By default a unique security code will be added to each pickup order for verification purposes.
Should you wish to disable this please follow the instructions below.
Login to Store Pickup + Delivery from your Shopify Apps page
Click on the Store Pickup option:
Scroll down to the Security and fraud prevention block and uncheck Enable pickup security code:
Now you’re ready to go! Customers will no longer receive a security code when you mark an order as ready for pickup.