Dear John,
I would like to point out that, if the software application is not tested in PostgreSQL on your laptop, it can fail on Heroku.
Today one of my team member approached me as she couldn't deploy our own website on MySQL. It was tested fine on SQLite. The reason was, she didn't listen to our advise to develop it on MySQL. She did it on SQLite for ease of use & build.
Please make sure that you test it on PostgreSQL (in your laptop) before planning to deploy on Heroku. Otherwise, this project may take more time and costs more.
Always use development environment identical (Except hardware) to the deployment server. Ie, use same version of OS, DBMS, Programming Language & libraries.
Regards
Varghese Chacko