We sailed last month, and I was SO stressed about the test. I booked an appointment with Quest Diagnostics using the link Royal provided. My parents sailed with us and took their test in Florida about 36 hours ahead of embarkation. My daughter and I got tested in Albuquerque 48 hours before embarkation. My parents had their results within 24 hours, but my results did not come in until 1am the night before the cruise. I was freaking out! As a backup plan, I was going to show up at the terminal and ask that they let us use the testing they have at the pier for unvaccinated children since in that case our test results would still not have arrived in time. But that wasn't necessary. I printed everyone's results from the hotel, and check in was very smooth.
I would likely choose this option again because it worked, it was free, and I didn't have to worry about anything going wrong since I got tested at a professional lab. And it was a PCR test as well, so it's extra accurate. Additionally I would say that I have a lot of confidence in the labs that do testing in Florida because they seem to know the drill when it comes to cruise testing.
And finally, another thing that gave me some comfort was I used a home test to confirm that we were negative before we went in for our lab test. It's bad enough having to wait for the results without also worrying that they'll come in POSITIVE!
I hope that helps!