Worth noting that this ancillary revenue also includes the sale of miles:
It’s also important to note that the figures in the report are not synonymous with passengers’ out-of-pocket expenses. They also include revenues the airlines earn by “selling miles” to credit-card companies and other corporate partners that want to link to their loyalty plans.For major U.S. airlines, those revenues are believed to exceed $6 billion or one-half of the group’s total.
If we assume that airlines sell to the banks and corporations at less than a cent per point, that is at least 600 billion miles sold last year.