Yeah a PO is for their own tracking purposes, it really means nothing to you as it's not a guarantee of payment.
You've done the right thing in not proceeding, if they've got such a problem with paying a deposit then the red flags would already be flying in my mind.
Our policy is:
100% for orders under $500
50/50% for orders over $500
Payment on completion for smaller long-term customers with good payment history
30 days accounts for established larger customers