[donna]

My suggestion would be to introduce payment by 1) allow a set of usage free (e.g. up to 10 forms) 2) create individual small payments for each use (e.g. after cost 3¢ ea. initial start charge is $20 until $20 is used up) 3) offer monthly subscription based on unlimited usage

Treat it similar to the Virtual Goods model.

[stulogy]

Interesting. I suppose you could buy credits, similar to Skype. I think people might get a bit frustrated though having to purchase additional project. I like the KISS ethos, so having a Free and Premium option certainly makes things a lot more simple.