How are you using "order", as a direct command (on the order of the police) or, as a promise to purchase (the company ordered 15 cases of beer)?
Per the order received from PXP, Z undertakes to supply the followng products. PURCHASE.
On the order of PXP, Z undertakes to supply the following products. PXP has demanded that Z do this task.
I'm not sure I'm using it in either of the senses you mentioned. It's an order in the sense of a company ordering a product from another company. One company is placing an order and another is processing it.
Maybe there's isn't a preposition in English that expresses that; I'm not sure.
You could also say, "In accordance with the order..."
Your suggestions don't exactly express what I'm trying to express, which is something like this: "whenever PXP places an order, Z will supply the following products." Is there a different way of expressing "whenever PXP places an order"? Perhaps "on/upon placement of an order by PXP"?