Google invoices with billing IDs AND invoice numbers

[tamacodechi]

While rewriting the specs I came across the fact that for google invoices we are recognising both the billing ID and the invoice number and we save whichever was recognised first. Is a billing ID a type of invoice number or should we not be recognising it? I have not run across a bill that has only one or the other, they always have both, so I can't really tell why we have the need for recognizing the billing ID 🤷‍♀️

My guess is there must've been a bill that did only have a billing ID, but it is not the one that the spec was written for.