I suggest to use an open ticket for that purpose. You can add a virtual user to open open tickets only. The tickets won t dissapear until they are paid.
I just got your message today. I don't use Facebook anymore but would like to discuss the above desired functions. I tried to private message you through this site however the link is bad. Perhaps we can just discuss what this would take openly here to help others that desire the same function.
Honestly for this to work for both you and many others there almost needs to be a customer system added. Something where you can track sales of regulars AND offer discounts and specific pricing. This would be a very large change, and require a good amount of code work and testing.