Authorize.net/ Website Payment Setup/ Changes and Updates

What we are wanting is to add payment functionality to our website. To make this clear, we do not want to add a payment option to the product portion like a shopping cart, rather we want to allow our dealer base to be able to log in to our website and make payments to already existing invoices. Our site has three ends to it: (1) End user which is what you see when you go to [login to view URL] ; (2) A dealer site in which if you click on certain areas of the site it takes you to a dealer log in page in order to access that information; (3) On the top right Order Online area, this takes them away from our site and to our inventory/ordering portal. Basically we are adding this payment option to the number (2) the dealer access only portion. As I said, we don't need a log in page created, as this already exists. I am attaching two mock-ups, one show the home page with the area added in green area on right where the link will be that allows them to "Make Payment Online." The second pdf shows what I would like the payment page to look like once they click on the home link and log in through the dealer log in page. I also will want a link within the Marketing Tools page, under "Manage Account" that says, "Make Payment." We have an account set up with [login to view URL] and we already have the SSL assigned to our site. I am sure there are most likely a couple other pages, but I wasn't sure if those are done through the [login to view URL] or us, I am speaking of pages like, "Oops, you didn't feel out all fields, please fill out and resubmit, and "Thank you" for you payment, please print this out for your receipt. Also, I am not sure if [login to view URL] has a logging system or if we need to create one through our backend admin. This would be another element within our site. We have a control panel (Admin) for ourselves that allows us to add dealer accounts, add staff members, add items to marketing tools section, and add closeouts. It's actually a pretty simple admin. Images [login to view URL] Please do not apply unless you can make these changes. Please explain how you would make these changes.