$PATH: Seemingly several definitions, and yet unable to modify it

For a while now I've been struggling to deal with my $PATH variable but up until now I've always given up on it. This time, I've had enough, and after scouring AskUbuntu I can't find anything that works. Here's the problem:

I wish to permanently add Postman to my $PATH. The file-path for the executable is /usr/local/Postman/Postman.

I have attempted to edit my path in each of the following ways:

• Editing ~/.profile: I have read on these forums that ~/.profile does not load if ~/.bash_profile and ~/.bash_profile exist. To save everyone's time, I can confirm off the bat that neither of these files exist.
• Editing /etc/environment: I've also read that I shouldn't use this file to modify my path if I'm not the only user on the system. Don't worry - I am.
• Running PATH=$PATH:/usr/local/Postman/Postman: This is the only method that changes the return of echo $PATH, but I still get Postman: command not found when I attempt to run Postman, and the changes are not permanent or applied to new terminal launches.

None of these methods permanently or effectively change the output of echo $PATH so $PATH is clearly being defined somewhere else that I can't find. Please help!

Currently running Ubuntu 17.10 but this problem has persisted since installing 17.04.