We use infusionsoft + the trycelery cart and are trying to decide whether to upgrade our shopping cart. (A single physical product, quantity-bundles + upgrades and accessories). I'm sure everyone loves their cart but does anyone have a been-there done-that where they upgraded to a cart that increased conversions (especially mobile), and offers full control over the cart's HTML/CSS/JS/? What is generally the best cart when considering conversions alone?
Considering the fact that you have a single product, I would have recommended Magento but it's way too much for what you need. WooCommerce is a great option for that.
I currently work with Shopify and Bigcommerce. Both have come a long way and have a great selection of apps that allow you to do all sorts of things. Depending on which plan you're on, Shopify does allow you to change a lot of code within your site.
Hope this help.