E-junkie.com
A simple shopping cart approach to selling physical or digital products.
Create a “Buy Now” button for each “product”. Add the code that e-junkie provides to a WordPress post or page.
All of the functionality happens in an E-junkie interface.
7 day free trial. Minimal expertise to set up.
Monthly pricing depends on # of products for sale.
You will need the API from Authorize.net to set this up properly.
(don’t be surprised by the animated logo that has eyes watching you on the website)

Authorize.net Simple Checkout program
Not a shopping cart.
Create a “Buy now” Button for each product. Add the code that Simple Checkout provides to WordPress post or page.
All of the functionality happens in a Simple Checkout interface.
Minimal expertise to set up.
Not useful when several products may be purchased together in one transaction.

WordPress plugin ideas

Gravity Forms Creator integration available with Authorize.net
This is not a full e-commerce system.
Purchase the developer package to get Authorize.net integration $199/year.

Woo Commerce
This can be a full e-commerce system.
Integration is available with authorize.net.
Annual pricing depends on your selected features.

Easy Digital Downloads(EDD)
Purchase the Authorize.net Gateway add on.
Pricing depends on your selected features.
(The blue smiling character representing the EDD brand is LOVABLE!)

Ithemes Exchange
For digital and physical products and membership sales
Authorize.net 3rd party add on is available.
Pricing depends on your selected features.