> The actual answer is there's operational cost to handling cash...

Really? From TFA:

  When comparing the prices of First’s weekly bus pass in my area, which is available to purchase on the bus, and its monthly pass, which is exclusive to its app...

If the reason were "operational cost to handling cash", you'd expect that cost to be substantially less for the one where you handle cash only once per month than the one where you have to handle it four times per month, no?