The easiest for me is to just get email and units. With excel, should be able subtract the numbers of units you think they used up based on their purchase date. So the database will start only with account records, no licenses. When the user uses DigDB in Excel, it will contact the server which will then subtract from units to create licenses.
I don't know anything about spreadsheets, so the best for me is to get the data as text. This would be a good format:
"fschmidt@gmail.com", 10
"bob@gmail.com", 22