I suggest taking a look at this git repository that has some good working examples. Also, log into the middleware and there are lots of resources there.
I have working APIs that connect to P21.
The GIT repository has transaction API examples. But , I am particularly looking at passing on Invoice payment detail to the ERP.
Any pointers for that?
Are you actually using v12.12 P21 desktop version? If so, holy crap. That software is like 10 years old, you want to push that client towards migrating to web version which has a proper API system. If you need help, let me know.