I’ll likely reply to the Invoices thread too.
Around customisation/interaction though we’d need to answer these kinds of questions:
What do invoices need to do? For who? in what circumstances and over what time?
How do these customisations currently happen? any why?
How does this differ across different global cases and which are critical to each global instance? e.g. I can see some complexity with delivery methods potentially in countries adding in new delivery methods.