We are running Interspire Shopping Cart 4.0. We are looking for someone to modify the way people pay us. By default, Interspire has an express checkout, where there are 4 steps on one page, or a standard checkout which is 4 steps on separate pages. We're looking to modify this to have one step on one page as seen in the attached screenshot. Customers should be able to enter their address and billing info on one page, see a preview of what they're paying for on the right, and choose a payment method. If they choose credit card, it should show a credit card form (like Interspire already does) right on the same page. If there's a problem with the credit card payment, it should either tell them on that same page.
Also, if a customer already has an account, they should be able to log in and have the option to select an existing address or enter a new one (Interspire already has this functionality). If they enter a coupon, it should show up the discounted price to the right without reloading the page and losing the information they entered.
See screenshot for details.