Most sole traders don't do their invoicing at a desk. They do it at 9pm, on their phone, after the actual work is done. So the invoicing should live where they already are: WhatsApp.
The concept: finish a job, send the agent a message (even a voice note) saying what you did and for how much. It drafts the invoice, shows it to you in the chat, and sends it the moment you tap approve. Then it does the part everyone hates: politely following up on unpaid invoices, and giving you a simple picture of what's owed at the end of the month.
Nothing goes to a customer without your say-so. The agent drafts and reminds; you stay in charge of every send.
The admin happens in the same chat where the work was arranged.