oh, don't bother - I thought just in case You have some.
May I suggest You add the weight of Your kits to the catalogue, since You are offering two shipping options based on the weight?
The short answer here is: worldwide shipping is $2.91 for one and $4.65 for 2.
The long answer:

The shipping is always a little confusing, especially when you order something small like this kit. It weighs 140 grams and so several options are valid.
When one airmails something, it can either go as a letter or a parcel.
I generally try ship stuff as a letter, not as a parcel, as letters are far cheaper.
There are rules for what constitutes a letter:
A medium letter must weigh less than 250g and be no thicker than 10mm
A large letter must weigh less than 300g and be no thicker than 30mm
So the shipping option that says "airmail in units of 300g" means "try and post it as a letter". Since a single model is 140g and quite thin, it can classify as a medium letter.
The "airmail as a single package" is the same cost up till 300g. After that, if you want it in a single envelope then it switches over to a parcel and those are weighed per 100g so there is a big price jump. Almost everyone who orders, even if they order a lot, take the "units of 300g" option unless they want tracking (hardly anyone does)
Incidentally the volumetric constraints mean the weights are sometimes a bit weird. The
cyborg sentinal weighs far less than the buffel even as a twin pack but the plastic crystals make it more than 10mm thick and so I have to artifically bump up the weight so that it appears as a large letter.
Sorry... that was perhaps more detail than you wanted
