Surely some one of you is familiar with what a mail-order company goes
through. This company has only a few products, but thousands of clients.
I need a Sales, Billing, and Receivables program to handle the thing,
but I need to be able to customize it myself, own the source, etc. Anyone
willing to sell me the basic stuff (in ANY development language) I'll
be willing to pay about $1,000 to. 

It has to be ready now. I need this sort of solution immediately. With more
time I'll just develop one myself. If you can have me a prototype in
two weeks, you can make some quick cash.