Billing systems are a nightmare for engineers

That's an axiomatic and therefor meaningless statement. Knowing something is hard to solve doesn't mean new tech won't bring improvements regarding the problem.

It doesn't follow that having an immutable ledger means it has to be used for every single application. That would be like claiming trains are useless because they can't climb stairs. They don't have to do that, we keep walking the stairs just like we've always done.

Immutability is great in situations where you want to record that something has taken place and don't want disputes about this. Some of the things you mentioned can be automated to some degree. Take returns as an example: You sell the item with a unique token that makes it identifiable and everything else can be automated, including chargebacks. We already have this to some extent with bar codes and RFID tags, but it can be taken to the next level with a non-fungible and non-falsifiable token that's connected to a smart contract which automatically returns money, does the accounting, updates the inventory, reports the tax data and so forth if authorized by the right person. As the owner you can easier monitor and analyze the flow of money, better detect fraud and come up with a myriad of improvements. Let's assume we have autonomous delivery trucks soon, then the entire supply process could be automated, including the billing as soon as the product is accepted. With something like the Helium Network shipments can be tracked, so there can be no dispute over where they are located.

For company owners who really doesn't want to spend much time at the office you could go a step further and even set conditions for disputes like contracts triggering a certain action after a suppliers products have been rejected too many times because of quality issues, or vice versa is a customer keeps ordering and returning products from a factory. The contract could switch to automatically ordering from a competitor.

The complexity in billing, invoicing, payments etc are _not_ the stuff that computers or systems do. The complexity is and always will be in the stuff humans do.

For selling a SaaS worldwide, Stripe quickly becomes just as much of a nightmare. Taxes, billing, chargebacks, subscriptions, web hooks, different payment methods, VAT validation, card expiries, legislation, changing laws, SCA flows, and my favourite: SCA iframes that become non-responsive on browsers in a certain language. It is a never ending source of edge cases.

Don't ask me how I know... My biggest mistake with my business was that I vastly underestimated how much work it is to accept payments. In my case, it proved to be more work than building the product itself.