Billing systems are a nightmare for engineers
Just wait until you meet billing's angry roommate: invoicing.In the US, an invoice is just a weird PDF that you might glance at before sending off to your accounts payable team. But in other countries, especially those that use VAT style taxing systems, an invoice can be a legal document that expresses costs and taxes in a legally prescribed way for accounting purposes. Many countries have prescriptive rules over how you invoice, who can write invoices, when you can invoice, what goes on an invoice, etc. And every country does it differently.Are you building a service that charges incrementally many times per period? Or even worse, refunds incrementally? You might be sending a truckload of expensive  accounting papers to your customer every month, one per transaction. And each of those pages was printed using government prescribed software from oddball vendors you must use or else.