You might get charged taxes, and the tax collection fee. Given that the handles are US$20-30, there is a good chance it'll sail right through.
I just got one of their M700 detachable magazine trigger guards a couple of weeks ago, and it came through just fine.
Are you going to be installing the handle yourself?
Installed a handle on one of their bolt bodies last week. I use the homogenized paste silver solder/flux from Brownells, along with a simple jig that I made to hold the handle in exactly the correct position while brazing. Heat with an oxy-acetylene torch.