Comment by mos2

There are services such as snipcart and foxycart that you can trigger via html and javascript. Like most non-locally hosted ecommerce solutions they send you to a subdomain for the final checkout. I have tested both on a static site and both work very well. Snipcart is easier to implement but lacks customization. If you are looking for heavy customization (like custom products beyond just sizes, colors and options) without having to build out your own system, check out foxycart.