Although it is tempting to reuse the same tables, In the long term I believe you'll get more flexibility in having separate tables for quote/orders/invoices+credit notes as theuy are functionally different and I can almost guarantee that they'll will evolve in slightly different directions over time. Also the reasons for browsing these tables will not be the same, as the people using them will be different quotes: sales team, real time; invoices : credit controllers sending reminder letters, entering and allocating payments; orders will be used by sales and people on the shop floor preparing deliveries etc.... they will have conflicting needs and trying to get the ONE schema that fits all that you'll end up with a lot of tricks, fields empty because we are in this or that format etc ... so you'll rely a lot on the front end and your memory instead of having a clean schema
The alternative is to use inheritance in tables but I lack experience with that to give a proper opinion. I myself am wary of the inheritance concept as I do not know what the long term practical implications are, even though it is conceptually sound.
Hope this /opinion/ (hence highly subjective) is useful
