Got doctor's notes from Rocket Doctor.ca and also from VirtualDr.Ca, for $20 each and tried them each on different trips since December. You need these to enter the United States, and customs agents don't check it - it's the airline check-in staff.
That said, every time I've managed to check in online or at a kiosk and gotten my boarding pass, I have never had to show any tests or doctor's notes to anyone. (This is a bit annoying considering the trouble/cost we went through to get them)