Because the iPad is a great device for designing t-shirts, I expect that the number of sellers who would like an integrated workflow that allows them to both design and submit directly from their iPad will grow rapidly.
However, the MBA website currently does not work on iOS (at least not on my iPad Pro with iOS 10). While the situation has improved and most buttons are now responding correctly, the last pop-up confirmation window (after hitting the submit button) still does not work (both the OK and the Cancel buttons remain unresponsive).