Having struggled over the years as a Miva Shopping Cart affiliate and then switched to the open-source alternative osCommerce for even greeter nightmares, I am pleased to say that there are better and much simpler solutions for installing a shopping cart in the WordPress CMS.
Free forever start up version for up to 10 products. No transaction fees. Then as you grow, $15/month for an upgrade that has more features including being able to sell on Facebook and Instagram.
Here’s an example of a very successful Ecwid installation: