What is the difference between UPS and USPS?

With Priority:

You can ship up to 70 lbs.

You get free delivery confirmation on online postage.

You can use free envelopes, boxes, tape, and labels provided by the post office.

You get to have "Priority Mail" all over the package.

With First Class, there's a 13 oz. limit and you don't get the perks.

The website might report an extra day for first class on certain long distances, but they travel together, through the same sort facilities and by air. In practice, they're the same speed.

Neither service guarantees times, but it's usually 1 day within a hundred miles or so, 2 days to most other places within the 48, with coast-to-coast sometimes taking 3. AK, HI, and territories usually take 3-5 days, depending on remoteness.

If a pre-paid, non-flat package was dropped off at the counter buried in Parcel Post, their red, white and blue packaging might help keep it from accidentally traveling with the Parcel Post, by surface. The people at acceptance are generally the only ones who pay attention to the postage or postage type and they group Priority with First Class.