The shell passed USERNAME as an environment variable to the embedded python heredoc (USERNAME=$USERNAME python3 ...), but the python block referenced USERNAME as if it were already a local variable. The result was NameError on every run, so the export script could never produce a cookies/<platform>_<account>.json file.
Fix: read the env var explicitly with os.environ.get('USERNAME', '') at the top of the embedded block, then use the local USERNAME as before.
DerekEXS
billy