MA has a law / tyrannical decree that lists which handguns a dealer can transfer in state.
MA defines a firearm , handgun and rifle, as being able to fire a bullet.
As a result, FFLs can legally transfer a frame, even a frame not on the list, because it can not discharge a bullet.
Once it is transferred, the handgun can be put together but then needs to be registered with the state, not registering is a crime.
Does the state prosecute people for working around the transfer laws by taking the gun apart for the transfer? Is that a crime?
I don't know.
I know of one gun shop that was selling new glocks ( not the on transfer list) in 2 pieces and were told to stop.
I do know they have prosecuted people with parts that made a whole firearm, even though it was not put together, for not registering the parts as a registered gun.
In this state I would be very leary of doing anything outside the bounds of what is specifically allowed ( without a conscious decision knowing what the consequences can be ) because it is filled with corrupt SJW judges and corrupt prosecutors looking to prove how anti-gun they are. And the juries are the same.
T-22 months and counting until my escape.
ETA: I do know that people legally build race guns from a transfered frame, but they are not whole guns taken apart and put back together. Does an XDS have matching serial numbers on any of the upper parts. That may be an important detail.
You can legally ship the whole gun or just the frame to the FFL. The law dictates what the FFL can transfer in state, not what the FFL can have. I would do nothing without talking to the FFL first or what ever you ship might get shipped back on refusal.