With most us postal services the initial costs are steep, but tapers for higher weights as you'd have the base charges simply associated with the shipping, plus whatever was required.
If Jetfire is the first item in your POL then $43 sounds about right. Future orders (if your POL is set up right) will have low shippin charges, as all you doing is adding Weight to an existing shipment. You'll be charged small shipping - between $5-$10 or even $0 depending on how much they add tithe total weight.