I have written a 3rd party billing application like this, for the domestic service industry, and the major hurdle you will have to get over is that merchant account issuers won't give you an account.

They get one whiff of "3rd party processing" or "payment service provider" and their fraud alarms go off. Certainly in the UK they (Major banks & Paypal) will not issue me with a merchant account, and the reason they cite is that it's too big a fraud risk. I can see how it would be an easy way to launder money.

If you manage to overcome this problem, I'd love to hear about it :-)