The only help i can offer for this is to point you to the "Creating a Digital Goods Download Center" tutorial on the eCart support page:
ecart/
the record sets that are created in this tutorial will not be exact, but if you have an understanding of MySQL, you should be able to see what is going on and adapt it to fit the power store 3 data base structure.
This is really not something we can offer support for. The option for digital goods has been added to PowerStore 4