Once you weigh a shipment and add multiple packages for a shipment, the scale should be deactivated for the package so it doesn't overwrite the value that's there already. Super annoying when you have multiple packages for a shipment and scroll through them only to find that the weights all get messed up.
Official response
Product Development
This has been updated as of ShipCenter 1.3.0.5! Here are the relevant release notes:
FIXED: Shipment Editor: Weight scale will not override previously saved package weight.
Check out the ShipCenter release notes for all the updates in our latest release!
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