What's the most painful part of invoicing you wish was solved?

Invoicing is one of those things that sounds simple until you're actually doing it every month. For me the worst parts have always been:

• Writing follow-up emails for overdue invoices and trying to figure out the right tone

• Context switching from actual client work into "accountant mode" just to send a bill

• Clients asking "can you resend that invoice?" because they lost it in their inbox

• Manually creating invoices every time I needed to send one to a client. It seemed like every time was the first time.

I'm building something in this space right now and I want to make sure I'm solving real problems, not just my own. So — freelancers, agency owners, small business operators — what's the one part of your invoicing workflow that drives you crazy? The thing you wish just handled itself?