can you describe why? (best actually in the commit message)

In any case I would rather like to see this as part of the config, with a default that is the same as what we want to run in production after we've agreed on it.

There are also other aspects to the invoice expiration that I didn't consider. (the expiration window allows you to limit exchange rate risks, e.g. when you send an invoice to a client)

I don't see an immediate reason to change something here.