Here's a guide on setting up flat rates for packages that can fit multiple items of the same SKU.
Flat rate (also called one rate): This is a service provided by different carriers in which certain types of packages, regardless of weight and regardless of shipping distance, will always have the same cost to the sender.
If a flat rate (one rate) package can fit multiple items of a single SKU, it's simple to create a setting in the system that will ensure the least expensive shipping option is chosen.
First, in WMS, you will go to Catalog > Catalog. Search for the SKU you want to modify.
Click the gear icon under the Action column to the right of the list. Click Flat Rate Option.
The Flat Rate Option menu will appear showing you the SKU details and current shipment settings (product dimensions, weight, box size, etc.)
Note: pay special attention to the Package Max Qty as it will have a default value of 1.
Click Add Flat Rate.
On the following screen, enter the carrier, select Yes next to Is Qualified, select the package from the pre-determined dropdown list (the package types can be created by following this guide LINK), and enter the maximum quantity of that SKU that can fit inside the package. All of these fields are required. When you're done, click Add.
Back on the Flat Rate Option screen, the Package Max Qty has changed from 1 to your new assigned amount.
Note: If you set Flat Rate rules for many SKUs, the system will recognize that separate SKUs can also go into a single Flat Rate package depending on the Max Qty for that package and the SKU settings.