There are currently several methods for tracking sales using GA.
- Goals – You can set up a goal and simply enter the page URL to track the number of conversions and the value of a goal.
- Setup classic eCommerce tracking via ga.js
- Use analytics.js for Universal Analytics conversion tracking
- Or go with the new Enhanced Ecommerce tracking via ec.js
If you are using a shopping cart script then you should check the documentation for that cart software, as most will have a method already integrated into the cart.