You aren't suppose to do it - but people sent me P-Mags in care packages and I just sent a couple to my cousin in Afghanistan.
Covered by ITAR and the USML
*(a) Nonautomatic, semi-automatic and fully automatic
firearms to caliber .50 inclusive, and all components and parts for such
firearms. (See § 121.9 and §§ 123.16-123.19 of this subchapter.)
I'd say mags springs fall under the definition of components or parts for a semi-automatic firearm, but I'd be willing to listen if someone has information otherwise.
As much as I sympathize with the situation, messing with ITAR is very bad juju.