No, that hasn't happened to me yet, nor have I heard of any issues with importing GBB mags. I would have assumed that if mags are prohibited, they would have confiscated my last eHobby order containing 4 Hi-Capa GBB magazines and 5 G3 AEG magazines. Pursue UNCompany (with the "help" of PayPal) and demand a full refund. No, that shouldn't have happened - I'm not sure what went wrong.
I'm sorry to hear that.