As you mentioned, the user 'daemon' is the one running services as PHP-FPM or Apache. However, we use the use 'bitnami' is the owner of the OpenCart app for security reasons. In fact, the OpenCart app is set with user:group 'bitnami':'daemon'.
Those services (Apache and PHP-FPM) 'read' permission for files and 'read and execute' permission for directories and that's the reason for the standard 644/755. If you want daemon to be able to write on directories such as /opt/bitnami/apps/opencart/htdocs/system/storage/ we recommend you to execute:
sudo chmod g+w /opt/bitnami/apps/opencart/htdocs/system/storage/
Anyway, we don't recommend you changing the permission configuration since it's set taking security in mind. If you want to install a plugin, we recommend to do it configuring FTP. You can follow this guide
If you want to modify the content of your OpenCart app's files, we recommend yo to connect via SSH/SFTP as user bitnami and apply the changes you need.
What are the permission errors you are getting?
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.