Hey Brian,
Those limits are for free membership and post these the supplier will have to pay for being able to transact. The registration process is not an exception to any subscription type to Ariba Network. Can you please help me with the link to details that you are referring to when you say "The network will deliver a PO to a supplier’s email address automatically", I think this should happen outside the network not through Ariba Network.
Regards
Vilas