Serial number counter

A customer decides what items they want. From a customer database a new job ticket is created and all of the items this customer wants are entered via a portal. As the items are entered, fields in the portal are populated with information from another database about the item such as price, color, size and category. The category may be wood, metal, plastic, glass, or paper. The order that the items are entered is critical for assembly at a later date. None of the categories are really important except for the paper one. What I need is a (auto entry) counter so that everytime paper is the category a (field named paperpostion) would populate with it's position (starting with 1) and increase by 1 everytime paper was the category. Like this

item category paperposition

metal

wood

paper 1

paper 2

plastic

glass

paper 3

The next customer decides the items they want and if paper is one of them, their first paper item would get a number 1 and so on.

A solution that seems so simple eludes me. Any help is greatly appreciated.