I have a site that sells trees and other items. I require that only 1 tree be bought per transaction. I also require that a tree must be bought every transaction. I don't want the checkout to go through if there is no tree is the shopping cart.

What I tried with rules is this:

EVENTS
Completing the checkout process

Conditions

Order contains a particular product
Parameter: Order: [commerce-order], Product SKU: Six Foot Tree, Operator: =

Actions

Complete checkout for an order
Parameter: Order in checkout: [commerce_order]

Could you let me know what I am doing wrong? It is not working.

Is there a simpler way to do this.