Adobe Business Catalyst offers bundled hosting & e-commerce. But you must host your sites on their servers.
If you don't want to change hosts, look for a 3rd party shopping cart.
UltraCart is PCI compliant & offers a 30 day trial
Here's a low-budget site using Paypal's idiot-proof free web Standard Shopping Cart.
See any of the Galleries and view the source code. Each button's code starts with my comment
<!-- ADD TO CART BUTTON -->
I just paste their code into code view with a DW "snippet"