At a high level, this is what your design will be:
1. You will first have a Gallery of Customers
2. You will set the OnSelect property of the Gallery to take you to another screen that has an Edit Form to display the selected Customer. So, OnSelect property of the Gallery = Navigate(CustomerDetailsScreen) and Items property of the CustomerDetail form will be CustomerGallery.Selected
3. On that same CustomerDetailsScreen you can have another Gallery of shipments filtered to the customer you have selected in the 1st screen. So the Items property of the Shipment Gallery will be something like Filter(Shipments, Customer = CustomerGallery.Selected)
This should give you all you need!
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Hardit(Haman)