100 Thoughts on Pricing: #43
Consider including a contractual obligation on the part of the client to promptly deliver POs where required by them for payment. At least then you have a basis to point to when charging the late payment fee when they say 'our payment is not late because it's out of compliance with our PO system' (which they entirely control).
And if a PO is truly required for payment, consider writing into the contract that your services (or subscription, etc.) will only start once the buyer delivers the approved PO.