How an E-Commerce Business Hired a Lawyer to Draft a Purchase Order in California

It's not clear from the question if you verbally committed to the order or PO. This is something that you can and should dispute with the supplier. The purchase order may contain the essential elements of a contract: an offer (the specific goods/services), acceptance (your verbal agreement if it was given), consideration (the payment terms), and intent to create legal relations (business context). Contract law generally recognizes that commercial agreements don't always require signatures to be enforceable. If you made a verbal commitment, followed by the written purchase order, this could constitute what's called a "course of dealing" that courts may recognize. That said, the specifics of your situation matter greatly. If you never agreed to the order or the purchase order contains terms that weren't part of your verbal agreement, or if there are significant discrepancies between what was discussed and what's in the document, you have grounds to dispute certain elements.

Annoying as this answer it, it really does depend. Some purchase orders (especially in the government contracting world) automatically become contracts once they are accepted and signed. If this is the other sides normal mode of operating it is not unheard of for them to be reluctant to accept a different contract in case it creates different or conflicting terms. At the end of the day there is nothing inherent in a PO that would mean it cannot serve as a binding contractual document, it just depends on the language in the PO.