As a co/founder and core architect of multiple PCI level 1 compliant payment processing companies in the last two decades I will add as others have that billing and payments is exactly like anything else which is "difficult". Once you spend enough time in the trenches dealing with the challenging technical issues your experience and creativity leads to the proper solutions that further enhance the system. Everything is just inputs and outputs yet it is what one does in between those "puts" which matters and thus where experience can reduce such challenges.