Jersey Post requires that Shiptheory specify precisely which products are in each parcel. In order to achieve this Shiptheory will automatically divide your shipments' products into the specified number of boxes as evenly as possible by weight. As a result the weight of each parcel will not be governed by your defined shipment weight, but by the sum of your product weights.
If you need absolute surety about which products are in each parcel consider sending single parcel consignments with Jersey Post in conjunction with Shiptheory's shipment splitting functionality.
If you have any questions or issues, please contact support.