You can view a tutorial demo of Authorize.net Virtual Terminal here.
http://www.fasttutorial.com/merchant/If you familiar with Paypal, it's the same, you have the alternate script option of passing the item name, description, and price, without leaving your site. Once payment is transmitted, an instant approval is returned, and you can direct the customer to a thankyou page. The other option is to use a shopping cart, and there are 100s of shopping cart that uses Authorize.net It very simple and you can be up within a day, running in test mode if needed.
There is no additional login or password for customers. Pass through, for Simple Integration Method. I use www.unitedbankservice.com and you can check sample of the script integration solution there.
You can accept payment internationally, but I believe only in US dollars only.