If the transaction happens on the same website, as the test, you don't need to do anything. The user has a cookie that attributes the version of the test to the revenue tracking in your testing software (Optimizely, VWO). Once you connect the Google Analytics with testing software, you should be OK to go. If this is not the case, then you need to setup two email subscriber lists, one for each version of the test. Then manually tag email links in the emails to attribute the ecommerce revenue directly to the version of the test.