You are not selling the consumable product anyway, you have to deliver the product to customer location to generate accounting entries. Create a warehouse location of type customer and create a delivery order to the location.

The only accounting transaction for consumable products should be for the purchase (Vendor Bills), updating an expense account.  You purchase pens or paper and that is an expense that reduces your profit.

If you really want to track the inventory and record what is used, then set the items up as stockable and you can do an inventory transaction to consume them (sorry, I am not familiar with openerp v6 so I can't advise on the details of how to configure them).