Currently after creating a customer, there is no means to create an invoice for them
After adding customer if you then add their details, address, phone, etc (which you naturally would after adding a customer). They click save. It takes you back to the main Customer screen.
You then need to go to invoices - add new - then find the customer name again (which is also a pain in the backside) - because it does not use smart find like it does in Invoices
Your request has found it's way to the KashFlow Payroll team although this sounds like a BookKeeping question. If I'm incorrect and have misunderstood let me know, otherwise I will pass this on to my colleagues to review,