The shipper has to be a licensed exporter, and you are good as long as the order is less than $100 before taxes/fees.
Some items, such as ammo, scopes, and guns for instance require further paperwork regardless of price though, and these are not likely to be shipped.
The kicker is the manufacturer also has to be registered with some dept. in order for their products to be shipped.
As far as Rimfire Sports is concerned you shouldn't have an issue with those mags.
There are certain things they wont ship, but they let you know before you complete the order.
Have fun.
Mark