What label/thermal printer should we use?

Although technically any printer that prints ZPL would work, we are currently recommending the Zebra ZP450, Bixolon, and Arkscan. 

https://www.zebra.com/us/en/support-downloads/printers/desktop/zp450.html

Here are other printers that we have successfully connected with

Zebra ZP-505

Bixolon SRP 770 iii

Arkscan 2054A

Printers that DO NOT work

Rollo x1038

Zebra 2844

If you do a lot of shipping, you might be able to work with your UPS sales representative to acquire with technology credits. Otherwise, they are widely available on Amazon, ebay, etc.