If I remember correctly, most of the actual delivery people for Fedex and UPS are "contract workers," not employees. Thus, to some extent, they have no set schedule and work when they choose. So, items that are priority delivery are going out with the first workers to show up, while all other items will only go out if enough workers show up that day. This explains why you often see multiple trucks working the same neighborhoods in more urban areas.
Further, I believe their pay is also affected by how many packages they deliver (or attempt to deliver), so faster delivery (leaving at property edge or signing receipts themselves) means more deliveries and means higher incomes.
All of this explains, in my opinion, why the USPS is often the best service. Most, if not all, of their actual delivery people are "employees" with decent salaries and benefits. The have a valued interest in seeing that their employer thrives.
Chuck
They aren't. Fed Ex and UPS people are employees of the company, not contractors (they may use temp workers at high volume times of the year, like now, but that is not contractors), and the idea that a logistics company like UPS and Fedex would allow having no set schedule and showing up when they wanted is quite frankly mind boggling. Both of them are known for their logistics (both companies have as far as I know a service division that sells their supply chain logistics to outside companies, act as consultants, etc). The way delivery systems work that would be sheer chaos, when they have to coordinate stuff flying in from someplace, getting offloaded, possibly flown or trucked another place. There is a whole branch of math called path theory that governs getting things from point A to point B, that governs delivery of packages and messages on a computer network. I know Fedex does hire contracting firms in some cases for final delivery, or they use USPS, but again that is a very different thing, those involve contracts with a firm that has SLA's with it and so forth.
I suspect what this refers to is that Amazon has its own delivery service, but it also hires outside delivery firms to deliver for them (as well as UPS, Fedex or USPS), lot of those 'white vans' are outside services. Again, though, it isn't having people working for them who come in when they want, do what they want when they want to.