You do count Sundays because USPS does deliver Priority Mail on Sundays.
In my business I mail an average of 7 Priority Mail packages and 49 small First Class packages with tracking a week. I have never had a customer complain about a late delivery or missing package when mailed Priority Mail. (not saying it doesn't happen, just that they probably aren't late enough for customers to complain)
Many times I go for a month with no complaints about late First Class packages but then sometimes I get 3 complaints a week about packages not being delivered within 20 days. I get about one package lost a month.
USPS tracking is a joke. Most times when you track a package it will show when it was sent from the home post office and that expected delivery is in about three days and then again there will be no other remarks even after a week or two until the package is delivered.
Upon complaining to the Post Office, the only reply I have gotten has been "that the Post Office is in the process of closing down many of their distribution centers, including our local one in New Orleans, in a cost saving effort, and that some mail is being delayed in the process and that the tracking system has not yet been updated." When asked how long this process will take, I can get no answers.
I have been in this business for 33 years now and the first 25 years I got less than one complaint a year so the mail definitely used to be handled better than it is now.
When you are mailing low cost items, no one want to pay more for shipping than the item costs so it is not practical for me to insure the items, nor is it practical for me to use UPS or Fed-Ex when they charge double the rate that USPS charges. I just have to figure in the losses as one of the costs of doing business.
Despite my problems with USPS, I still believe that they are one of the best values around and much better than mail systems in other countries with the possible exception of China. I find it hard to believe that I can place an order with a company in China for a small item that cost 25 cents and get free shipping and it is delivered 4 weeks later. If we mail the same package locally it cost me $2.04 just for postage.