the php code of eCart generates an xml request that is sent to USPS.
eCart uses the V3 rate request API which is not supported on the testing server, you will need to use the production server address, and contact USPS to have you account updated to use the production server.
if it is still failing, open the shipping failure page, and go to the bindings panel, there will be a USPS Binding group, expand that and add the Full Request and Full Response bindings to the page, this will help us debug the cause by showing the response coming back from USPS.