Invoicing/Custom Payment Gateway with Wordpress

Hi all,

Because I'm so happy with Gravity Forms, I thought I'd ask the savvy people here if they know of a way to point customers to a web address on my wordpress site where they can input their payment code and pay their bills online?

I'm currently reviewing WP e-Commerce but I can't imagine how I'd design a custom bill payment system with it. Any ideas? Could this be a Gravity Forms feature in the future? Appreciated.

EDIT: I currently use Billings (Mac) for Invoices which I'm happy with. The idea is that I'd put a little blurb on the bottom with the web address for online payment. They can input a payment code there.

Yes, I have. I'll take a look again. Gravity Forms only supports FreshBooks currently, correct?

I definitely don't have enough clients to justify $20/month however (and that translates to more in $AU).

I do want all my clients on the same system however, and I have more than 3. I am happy with Billings, but to get it synced with my other Macs via Dropbox can be problematic.

The last time I looked, I didn't like the Freshbooks widget.

Is there no way to have some sort of an online payment system (not just paypal) without the ongoing Freshbooks expense?