Airside Post Office at DEN?
 

I'm in a bit of a bind. I'm traveling on UA through DEN tomorrow and I'll need a post office. The web site notes that there's a post office on level 6 of the terminal, but it doesn't mention if it is airside or landside?

I assume this means level 6 of the Jeppesen (landside) terminal. Way back when, agents at the Red Carpet Club could mail letters, I'm not sure if they still do now. It won't hurt to ask.

The website does state that the post office is in the Jeppesen terminal. Is that strictly landside?

Unfortunately, I need to purchase some packing material and mail a package so getting every thing done right will need a post office instead of a UA CSR.

The Post Office is land-side, but right above security, so if you've got enough of a layover it shouldn't be a problem to mail your letters. IIRC, there are some mail slots on Concourse B (UA), but no place that sells the supplies that you are looking for.

If you do go land-side, consider using the bridge checkpoint. Lines are much faster, especially if you've got status.