Hey StR!
Thanks for the quick reply.
I definitely get your point about wasting space and needless syncing. I don't want them either.
However, this is a little different. What I do is, when I purchase something online, I create a folder in the inbox with a name or a reference to that order, and when a new email comes in on that order I just drop it there. This way I can stay organised and keep track of what I am expecting (if I place many orders at once). Once I receive the order, I just move the folder to trash. So it's a temporary folder.
With Gmail those folders in the inbox are immediately visible and quickly accessible as you can see from the screenshots I've attached. They are above the inbox list of e-mails.
That's what I wanted in AquaMail too. To have an option to display folders inline with emails when you go into some folder. Keep in in mind that index in emails is also a "folder", so only folders that are subfolders of index will show up. All your archives that are no store in subfolders of index won't show. Besides that of course, Gmail does not sync until you go into that folder, so the space is not wasted.