I know that I can ship it myself but it would need to go overnight via FedEx, which is pretty costly.

Anybody know of an FFL in Bergen County that will mail it at a "slight" upcharge?

Also, can the gunsmith ship it directly back to me or does it need to go back through an FFL?

Check out https://www.shipmygun.com/go/, offedred by Bud's Gun Shop, I have not used this service, and as far as i know the gunsmith can return your firearm directly to you

But it needs to be sent via overnight/next day delivery if I'm sending it - right?

And the gunsmith can ship it directly back to me?

The overnight thing is not law - that's just a policy that UPS & Fedex have.  I know Fedex has a list of manufacturers that have a waiver with them that allows 2nd day shipping instead of overnight (and I'm sure your gunsmith will NOT be on that list).

You COULD, legally, ship it via UPS or FedEx by ground, by not telling them what's in the box (that is legal, because you're shipping it to a licensee), but you will be screwed on any insurance claim should they lose your gun ("You didn't tell us it was a firearm, so we're not going to pay on your claim").  Your choice...

And yes, the license holder (gunsmith) can send the firearm directly back to you (and he can use a Priority flat-rate $7.95 shipping box through USPS, because he's THAT special).