At the time e-tickets were first implemented in the 1990s, this simple data structure was not a major problem because most airlines still practiced product bundling.
That is, it was simply expected in most major markets that the price of a ticket included many other ancillary services like (1) checking up to two regular-sized bags and (2) one or two in-flight meals.
During the 1990s and 2000s, many airlines became financially distressed due to numerous factors: deregulation, competition from low-cost carriers, rising fuel prices, and economic shocks in the form of the dot-com bubble and later, the Great Recession.
Low-cost pioneer Ryanair became notorious for raising ancillary revenue in the form of unbundling every possible component of airfare and adding it back on through optional fees.
To facilitate these new business processes, airlines either have implemented nonstandard proprietary mechanisms (such as when selling tickets through their own direct-to-consumer Websites) or are tracking such transactions on paper forms, coupons or vouchers.