Do I need to print a shipping label before going to the post office?

No. They will weigh your parcel for you, tell you what you owe depending on your shipping method, and affix a label. The post office still serves lots of low-tech people and even people like me who, now and then, still mail actual letters with stamps on them.

Some post offices also have self-service machines that you can use to create shipping labels.

Things like printable labels are offered as a convenience, not as a requisite.

Are you certain that the USPS will properly handle the live plant delivery? That’s something that I would not normally expect.

Aside from that, and to your question, whenever I ship I print two labels: one for inside the box in case of damage to the container or the outside label. And yes, it’s best to have that printed in advance. Generally when I get to the Post Office there’s a line, and I want to get in that line ASAP, to reduce the wait.

It’s not necessary. It sometimes does take me a while when I am shipping to France but my local PO has always been very helpful.

Thanks for helping me handle basic adulting, guys!

@CWOTUS, yep, folks exchange plants and even aquatic animals like snails, shrimp, and fish in the mail all the time. I’m shipping a hardy species of aquarium plant that will do just fine in the 3–5 day shipping offered by priority mail.

As already mentioned above the post office will print out the label for you.
What I find incredible is how the algorithm figures out the address with only the house number and the first letter of the street address! That just doesn’t seem possible but it works!
If you use the self service machine you can play with it and see.
Fun!