On a purchase order an item that reduces the amount due to the vendor should be posted to what account?

Welcome to @CarNationAutos,

Are you referring to an item discount? If so, you can use the income account to keep track of the discount. Let me walk you through how to create it. 

1. Go to Gear, the Products and services.
2. Click New, then enter an item Name.
3. Scroll down towards the Income account drop-down, then choose Discounts.
4. If you don't have this type of account select Add new.
5. Hit Save and close.

With these steps, you'll surely record your purchase order and the discount correctly. You can learn more about the accounts payable workflow at this link here: How to group Accounts Receivable or Accounts Payable account types. 

We also have a guide on inventory and projects that I'm sure you'll find helpful. It has articles and steps related to vendor and inventory item issues.

That'll do it. Should you have any other concerns or questions, don't hesitate to comment below. We're always here to help. Thanks for visiting us today and I wish you have a wonderful week ahead.