When your shipments are exported with Parcelforce, they require that the weight of the parcel exactly matches the weight of products within the parcel.
By default Shiptheory will calculate your parcel weights based on the overall shipment weight. This can be changed so that Shiptheory calculates your parcel weights based on the sum of a parcel's contents.
To change the default setting view the Advanced Settings section at the Parcelforce settings page. Select Calculate from parcel contents from the Parcel Weights setting. Select Save to save these changes.
If you have any questions about how this works, please contact support.