The Drupal Web Content Management System is one of the best systems for E-commerce.
Smaller projects use the PayPal "Buy Now" buttons or a PayPal store. Churches and other charities have dedicated PayPal accounts that offer "donate" buttons. This all fits seamlessly into my Web Sites.

Larger projects use "Drupal Commerce," created by “Commerce Guys.” Drupal Commerce offers every thinkable possibility to set up an E-commerce Web Site offering almost everything you can imagine like:

  •  a web-store selling products with different options such as color and size: complete with choice of payment gateways; national shipping calculation; international sales and shipping calculating; automatic stock control and more.
  •  a Web Site selling services by subscription with a payment gateway and auto renewal.
  •  a Web Site for a charity accepting as well one time donations as a monthly fixed amount.

And much more. The only limitation is you imagination.
The E-commerce site can also be designed in the Drupal Web Content Management System and therefore fully maintainable by its owner.