We've found that Quotes, Invoices, Work Orders and Packing Slips all stem from the same foundation of an Invoice. They all show/hide different pieces of an Invoice to the customer.
Here's an example: a Quote is an Invoice with the word "Quote" at the top so the customer knows they are not required to make a payment at that time. A Work Order is an Invoice but with no pricing on it, etc.
When you initially create a Quote/Invoice it gets assigned to the 'QUOTE' status. This will show a 'QUOTE' marking at the top right for customers to see. Once you change the status to anything other than 'QUOTE', that marking will be gone and you will now have an invoice.
Tip: You can customize your statuses on Standard and Premium plans.