Honestly, the startup I work for got into the Managing Subscriptions business with their product and it has its own bag of hassles. If I add a seat in the middle of the month, do you bill immediately and prorate it, or wait until the next billing cycle? And if you do bill immediately, what if I delete a user and then add two more, do you bill me (prorated) for the two users, or only the one extra "seat"?

I'm not saying that you shouldn't go that route, just that sometimes more straightforward is better, sometimes knowing that I have the Deluxe account and if I want to add the Downtown office I'll have to upgrade to the Premium account makes it easier to understand. shrug.