[keefe8]

It looks to me like "First Class Parcels" includes Priority Mail, since there's no other category that could reasonably cover Priority Mail ("Package Services" can't since it shows much slower delivery from one side of the contiguous US to the other than is advertised).

I used to ship a ton of First-Class parcels and Priority parcels about 10 years ago, and learned that they had roughly the same delivery speed (2-3 days anywhere in the contiguous US) with First-Class just having a higher chance of delay.

As I never paid attention to letter delivery speed back then, I didn't realize that letters are much slower (except local). So seeing these maps today is informative.

[wsh]

The maps show the standards only for services, such as First Class Mail, that are classified as “market dominant.” Priority Mail and Priority Mail Express are “competitive” products.

[sokoloff]

If you click the text in the second paragraph at the top (that doesn’t look like a link), you can find the projected transit time for a specific shipment and service.

[dawnerd]

I did a ton of eBay up until this last year and there isn’t much difference between first class and priority other than priority gets well priority during sorting but usually they all end up together. Important to note for those unaware that first class parcels are limited to up to one pound.