I think it's time to bring an invoicing facility so we can be paid for expenses. Though clients can now manually pay for expenses, but a simple invoicing system could make it easy.
Now on Elance similar invoicing system is available. You might suggest to add manual hours. But I think it's not the right way to bill the clients even if they find it easy for them. I want to send an invoice for them to approve.
Yeah... Maybe. I mean, you're right that nothing exactly like that is built into Upwork... But there's not a lot of call for it. Most Upwork contractors don't have client-specific/project-specific expenses that we want to invoice.