It ain't the same
Keep in mind that I've been away from handling the transport of U.S. Mail since 1976, and could likely be totally wrong, but back in those days, we had air mail and first class mail... or in the vernacular of the airlines... green mail and airmail... the FC was in green bags and the airmail in orange.(There was also mail in red bags that was another class... don't remember what class it was, but I think it was below the green mail.... may have been bulk mail and magazines, etc...)
I think there was a difference of about 6 cents in the postage between air and first class... more often than not, the FC(Green mail) was on the same flights as the air mail... it paid different rates to the airlines, but still flew along side the airmail.... today a great majority of mail probably goes by truck.. the priority mail, first class mail and what ever the deferred mails are called today are most likely all on the same truck... only difference is, we as patrons of the post office pay different rates for the individual service...
personally, since this is more of a hobby business than any real profit and loss business for me, I'm not overly concerned about a few cents or a dollar on the cost of shipping... I feel that I've built enough into my pricing to absorb a small percentage of error in my shipping charges... for those of you who do this as a real profit and loss business, the shipping costs are much more significant and I understand the concerns.
Now you have 1st Class Parcel service, priority mail parcel service and parcel post.
Parcel post is almost never the cheapest way and never by more than a few cents...
Ist class parcel service and priority mail are both said by USPS to be 2-3 days for delivery.
One advantage to priority mail is that the boxes, envelopes and padded envelopes are provided free for the asking by USPS. If your local post office does not have everything you want you can go to USPS website and order it. Mostly it is free unless you buy prepaid envelopes.
Insurance is expensive .... but with confirmed delivery I have not found it to be necessary. I've been shipping for 3 years and have no lost shipments in the USA and only one damaged to the point where I needed to replace the items so I self-insure up to $100.00.
Both 1st class and priority mail are trackable with delivery confirmation but you can pay extra and get better tracking. Might be worth it if you are shipping only a few items, but not to me, shipping a fairly large number of parcels.
Priority mail is now zoned, but also has small flat rate boxes, medium flat rate boxes, large flat rate boxes, flat rate envelopes, padded flat rate envelopes, regional rate boxes and a lot of regular priority mail boxes. Sometimes for zones 1 and 2 and possible 3 will go cheaper than the flat rates.
Printing labels on line can save a few pennies but they did not make it as simple as they could have. It's not great for international though, or at least wasn't the last time I tried it.