I have +10 years of experience in Python programming.
Python is best for such scripting tasks.
I had many syncing projects before here on freelancer. Take a look at this for example:
https://www.freelancer.com/projects/6524595.html
I have worked with tens of REST, SOAP, RPC, ... APIs before.
Can you provide more details about your POS vendor and your website API please?
Thanks, Helmot