First, we don’t want you to have to wait for the Core Rulebook (and possibly other previous releases) until we are ready to ship the main product and stretch goal items. Second, it actually will not be much more expensive, because of how the prices for shipping are set. Sure, sending two packages below 2 kg is more costly than sending one weighing about 4 kg, but not by very much since 2 kg is something of a magical limit in this regard.
As an example, sending two packages weighing just below 2 kg each to the US will cost about 260 SEK; sending one package weighing just below 4 kg costs 235 SEK. Sure there are variations depending on where the parcels are headed, but we hope you will find the extra cost acceptable in light of the reduced product prices and quicker delivery of the extra books.