My friend asked me to remove a pinned/welded muzzle device from an upper and replace it with a new one then redo the pin and weld job.  He now lives in TX and I'm in OK so it has to be shipped.

The work is no problem, but would I need any type of FFL to work strictly on an upper half?  He'd keep the lower at home, of course.

Logically, I think I'm in the clear and I read through the NFA guidebook but I'm still not positive.

I think this snippet affirms my belief- https://johnpierceesq.com/do-you-need-an-ffl-to-manufacture-and-sell-ar-uppers/

The lower is what's registered, so you're good to go with the upper.

Perfectly fine to ship uppers.