I can't find it now but MSP issued a guidance a while back addressing this. The upshot was that even if you have the >10 round magazines shipped out of state to pick up, you're still purchasing while in Maryland and thus, breaking the law. I might have read it wrong, but that's what I understood.
Here you go:
https://www.mdshooters.com/attachment.php?attachmentid=216228&d=1508193978
It has to do with transfers by dealers, of >10 magazines out of state, and not with the actual purchase in state. Completely different question which we really don't want asked of MSP to get their "opinion".