Originally Posted by Cymro

The general rule with promos is that they stack, but they add, and don't multiply.

For instance, if you were normally earning 2 mile/$ but you had a 3 mile/$ promotion, for a total of 5 miles, if you then had the current double miles promotion, you'd get an extra 2 miles, not 5 miles/$ for 7, not 10, in total.