It would be really helpful if I could provide a manabox.app url to import a deck. Right now I have to download from manabox to a text file (which unfortunately doesn’t include the printing info) and then import from that text file.

Seconded. Manabox is a supported .csv import, but it would be neat if it was also a supported URL when creating a new deck. See error in screenshot.
Even better would be if the deck in moxfield could keep a reference to the deck in manabox so that when the deck gets updated moxfield can see it automatically.