Customers don't purchase since adding USPS shipping module [urgent]

Free Shipping Options freeoptions should have the Total >= 0

and a Zone Definition on it where in the specific Details of the Zone Definition you have set:
Country: United States
Zones: All Zones

that will include all of the United States for the 50 Sates, DC, US Islands, US Territories and Armed Forces ...

Then, you need to make a Zone for the Rest of the World that include All Countries and All Zones for those countries and add this Zone Definition to USPS shipping module ...

Next, in the Modules ... Order Total ... Shipping ot_shipping you would set the Free Shipping amount to 300.00 ... and make sure that it is enabled ...

If you set to Both, this will trigger for US and International at $300 on the Order ...

If you set to International, this will only trigger for the International Orders on $300.00 ...

It sounds like you have made at least one Zone Definition, but that you have not gone into the Details to define what belongs to that Zone and that you may not have made a Zone Definition for the Rest of the World ...

To build a Zone for the Rest of the World that is not the US, I belive post #5 can help you on this thread:
http://www.zen-cart.com/forum/showthread.php?t=39978&highlight=shipping+country+set

Then this Zone that is Not US can be added to the USPS shipping module and only those outside the US will see it ...