I am just back from 7 days in WDW (first vacation there since two years... finally!). I have benefited a lot of the information shared in this thread (and some other threads). So I like to report about my COVID-19 testing experience before my returning flight, more specifically with BeeperMD. In short, I could not ask for better, it worked very well for me.
I arrived at WDW on Saturday Oct 16. The day after, I completed an appointment request on BeeperMD website for a PCR test. I gave the number of my hotel room (Pop). Because my returning flight was scheduled on Saturday Oct 23 at 9am, I asked for an appointment shortly after the opening of the 72 hours window, that is, Wednesday between 11:30am and 1:30pm (the website offers 2-hour time slots). I thought that it would provide a degree of freedom of 2.5 hours in case where the actual appointment was going to be scheduled early. Once I had clicked to complete the form, I received an e-mail asking to complete an additional questionnaire with questions about my health condition, which I did immediately.
On Tuesday, I received a text message indicating that [name of the person] was going to visit me the next morning at 10:30am. (There was a phone number where I could call whether the appointed time was problematic.) I didn't receive any follow-up message. The person came to my hotel room around 10:50am. She apologized for being “late” (frankly, a 20-minute delay was the last of my concerns!). She appeared to be a very enthusiast and positive person. She reviewed my ID information, the content of the health questionnaire I had fulfilled, and she contacted a nurse through her mobile. That nurse had two or three ID questions to ask me. Then I was tested. The whole process took about 10-15 minutes.
On Thursday afternoon, about 30 hours after the visit, I received a text message alerting me that my test result (which was negative) had been sent to my inbox.
In contrast with information provided here, it didn't cost me anything. I was by myself, so I expected a $15 fee. I don't know whether the rule has changed or whether this no charge was a mistake. So the test was free and convenient. Nonetheless I left a tip to the person who tested me.
At the airport, I had to show my test result when I checked in with Air Canada. I didn't have a printed copy. I believe most people who checked ahead of me also shown their electronic document on their mobile.
I had an alternative plan to visit a nearby Walgreen with a cab on Friday morning, in case of problem. Fortunately, it wasn't necessary.
Once again, I want to acknowledge all previous posters who have shared information about this process. It was extremely helpful.