Looking for invoicing plugin recommendation

I haven't seen anything like this on WPMU, so I thought I'd just ask to see if anyone here has any ideas.

And quite honestly, I guess it doesn't have to be a WordPress plugin. I would be happy with any product that would work in a LAMP environment. But I'm not a PHP programmer, so it needs to be a turn-key solution.

I am looking for a way to send an "invoice" to a customer and allow them to just click a link to pay their bill. My preference is something that will integrate with Paypal Payments Pro. I can also accept straight Paypal as a secondary payment source, but absolutely need a way to accept payments that does not require a business to log into Paypal to make a payment. Right now I use OpenCart for my shopping cart, and I created an "online payment" product that can accept payments in $1 increments, so if a customer owes me $1200 the customer can log in and enter 1200 as the quantity. The downside to this is that the customer can make mistakes, and also it doesn't allow for fractions of a dollar. So I'd really like a way to just send a link, and when the customer clicks they have a pre-filled "cart" to pay $1272.50 or whatever.

I also have 2checkout.com, but they are not my preference because of how long they take to pay, and how much they retain in reserve for 90 days. I currently DO use 2checkout.com for the Membership plugin, but one of the reasons I offer credit card payments to customers on invoices is because it generally gets me paid more quickly. 2checkout generally negates that benefit. So, Paypal Payments Pro (which is also known as Website Pro, and I think can be used in Payflow Pro) would be my preferred payment gateway.

Anyway, does anyone know of a good solution for this?