We’re really not set up to debug complex scenarios, but a few points:
Are you running this against the Live (published) site? Or against the Preview site? In any case, make sure you do Save as that seems to matter.
What is the full URL? (you can just use wix.com instead of your real domain)
When do you get the error to use “use” instead of “post”? During execution? Or is it just flagged in the source code?
Why are you using use_myFunction() instead of post_myFunction? Did you try post?
It seems like a strange issue, but I’m sure that in the end it’ll end up being something simple. Have you tried the two examples that are available? Both are set up to provide a good starting point and they work great.