I received a form from USA Social Security concerning my current status.
I would like to use DHL to ship completed form back to them but they
use a PO box address. I called them and they said they don't have a
non PO box address. Must I use the THI/US postal service?

what about FEDEX......have you never heard of them ?????????????????!!!!!!!!!!!!!!!!!!

I've used ThaiPost/USPS for a bunch of stuff this past 6 months with no issue.

Just send it back by registered mail. It will be tracked throughout the system so you will have evidence of it being delivered. Cheaper than the express services also

Send it DHL to someone you trust in the USA and let them post it on to the PO Box for you

Don't think any couriers can deliver to a PO Box as the post need to be signed for.(could not when I tried)

 

Alternative use the post office registered mail, I think the mail can also be tracked.

 

Registered – Thailand Post

I use registered airmail - cost ThB 325 and traceable with the "RO" number on your receipt through:

Leaving Thailand: https://track.thailandpost.co.th/

Delivering in U.S.:  https://tools.usps.com/go/TrackConfirmAction_input

Thank you all. Looks like post office registered mail is the way

to go. Special thanks to mucat. I didn't know I could use Thai post 

tracking with USPS tracking.

I use the Thai registered mail also and their tracking system. Works great. Funny though I tried to access the USPS tracker (the second link above) and I get  this: 

You do not have permission to access this page
If you have found this page in error, please contact support

Weird. I use a VPN set in the US.

Hmmm - you may try to navigate to the USPS site on your own as the link does work for me on my desktop computer.

 

You can even sign up for updates - or for that matter have updates sent to your addressee - e.g. a birthday card.

 

Fax it!

I asked about faxing. They were not confident the the form would ever be read if received via fax.