[cperciva]

Not all SaaS includes free plans. I'd include the following two options as well:

5) Amazon approach - your usage gets added up over the course of each month, and your credit card gets charged at the end of the month.

6) Tarsnap approach - you start by making a payment to put money into your account, and this money gets used up as you use the service. You can add more money whenever you want. If the money in your account runs out, you lose access to the service.

Obviously, I like the Tarsnap approach. :-)