I am trying to figure out which solution (Simple Checkout, SIM or AIM) to use for my company which wants to allow its customers to pay their bills online. The amounts are variable and can change from customer to customer and from month to month. Simple Checkout would be ideal as there is only one item and we don't want to store any credit card or customer information. In fact, the donation feature is the perfect solution except on the payment screen the amount field is titled "Donation Amount" and it would be confusing to our customers to use this. Even though it's more robust than what we need I'm thinking that SIM is the best solution. I've read through a lot of information but I can't tell for sure if it meets our needs or not. We just want the customer to be able to enter their name, address, customer number, invoice number, payment amount and, of course, their credit card information. Is SIM the best way to do this? If so, then do I need to look for a shopping cart as well to handle the bill payment input? Is anyone else processing variable bill payments and if so what shopping cart and/or authorize.net solution (Simple Checkout, SIM or AIM) solution are you using?

Thanks in advance for any help or suggestions ...