e-commerce plugin

I am trying to setup the most simple e-commerce site ever. Only 1 product and that’s the only purpose of the site is to let ppl pay for this item online. I can’t seem to find a plugin for this. If possible I’d like for the product page to have a checkout link undre the add to cart button or even send the customer to checkout after they choose a qty then click add to cart. I’m using the gleam theme by elegant themes and not all the plugins work well with this theme. This is giving me a headache any advice is appreciated.