GPS has a Fair Usage Policy which restricts the maximum number of concurrent web service connections per client (IP address) to 20.
If you send more than 20 concurrent web service requests to GPS, an HTTP error message 403, sub-status 501 is returned. This can be mitigated by controlling your outbound web server configuration. For further details, please contact your Implementation Manager or Account Manager.
Note that in the Production environment additional charges may apply to web services that retrieve the balance on the card from GPS. Please contact your account manager to discuss. You can avoid this charge if you use the EHI data feed to maintain the balance in your own systems.