I've worked at two companies in their payment systems. This post does a good job of mentioning many of the billing system challenges. I think it understates how difficult and important idempotency is to maintain at scale with multiple teams though.

Additionally, it didn't mention financial regulation changes. India had changes sometime in the last few years which required whole new systems built that were specific to India customers. One example of complexity is this: does the system apply to customers who are in India or does it apply to businesses who are in India (the owners might not be)?

He also did not mention fraud, which is basically uncollected spend that results in charge backs typically. If you get too many chargebacks, you'll get fined. If you get even more chargebacks, you'll get kicked off the card network and will no longer be able to accept cards within that network.

Post pay (customer gets the product and pays afterwards) models are subject to more fraud, uncollected charges, etc.

There's also the fun of dealing with downstream payment processors. I haven't worked with a processor or bank yet that was very reliable. Sometimes they'd return 500 http codes and then process a payment, requiring manual intervention. Sometimes that happens with files of payments because most banks deal with files for transactions.