The version of eCart that you have includes the checkout wizard, so you can create a checkout that will accept credit cards directly on your site. If you have Universal Email installed it will also create the receipt email for you.
As for the page that the user enters details into this should be a page with an add to cart server behavior on it. For the price you can set the value to a text input so that he user enteres their own amount. You would also need a column for the invoice number.