Mags are legal to import and the majority of the time, it isn't a problem.
That being said, I imported 4 MP5 mags from the US last year. They got held up in customs for over a week. And in this time, the customs officers decided that each one of my mags needed to be disassembled completely. In the process, they lost one of the pins that holds the mag together. And they didn't even attempt to reassemble them. They just stuffed everything into a the box, wrapped it in a plastic customs back and yellow customs take and sent the mess on its way.
I've ordered mags from out of country many times though, and this was the only problem I encountered.