I don't find this to be always the case, there's many relationships were the nature of work cannot be established beforehand and we have open accounts, if you ever received an invoice you were part of such an account.

The proposal is simple, if you ever need a service and you request it of me, I will send you an invoice. It is implied in the request of my service that I would get paid, absent any negotiation for payment, I will send my best estimation.

Similarly if you download my software, absent any license, I have the right to send you an invoice. The fact that there's a license that explicitly mentions this is a nail on the coffin on the part of Oracle, but even without it there's quantum meruit.