Let's clarify first because I don't think it's clear and people are misunderstanding.
Scenario 1. you are buying a pistol from OOS, having it sent here in MD for the transfer and then want the LGS here to ship the +10 mags to someone else OOS? And you keep your gun here? You said "Transfer" so that's why I mentioned out of state.
#2 or you walk into your local gun shop here in Md, purchase a pistol(and providing that LGS pistol has 15 round mags in that pistol), have the LGS send them to someone out of state?
Sounds like you you are a MD resident purchasing a pistol that has 15 round mags. You'd like the local gun shop to ship them to a FFL out of state and then you drive out of state to that gun shop and take possession of them and come back home here?
Sounds like an awful lot of work for mags you could probably just go into PA or DE or VA to buy. Like the rest of us do.