There are some great options for adding eCommerce to an existing website but all involve a variety of considerations
1) How many products are you selling? If it’s just a few, you can skip custom cart development and use PayPal buttons. Make a great product page, and get the code from Paypal. No worries about security certificates and it’s easy to add new products just by adding new pages and generating new buttons. The major downside of this is that there is no “customer management” built into the website and PayPal’s merchant fees are high on a per-transaction basis. But it is by far the easiest way to set up eCommerce on a site. We also set up a payment page for consultants so their clients can pay online through their website.