Is the comma needed in "additional, custom apps"?
with additional, custom apps within the platform.
I agree with the comment by @Spencer that it's a good idea to provide the original text. However I also agree with @Yosef Baskin that the comma makes a difference.
... with additional, custom apps within the platform.
This means that the apps are both additional and custom. You don't have any yet, but you can buy them if you think they will be useful.
... with additional custom apps within the platform.
This means that you already have some custom apps but that you can get more custom apps if you wish.