I have checked you shipping settings, and as far as I can see you have successfully setup USPS shipping method. But there is an error in your origin address that causes this behavior.
Let me explain how the carrier calculated shipping methods (including USPS) in Ecwid work. When a customer makes an order in your shop and proceeds to checkout, Ecwid queries the carrier's site with the details of order and your settings and enabled methods for that carrier. The carrier's site then responds with some rates and Ecwid displays them without any changes.
In order to calculate shipping cost, carrier’s site need to know “from” address (you can set it in Ecwid control panel → System Settings → Shipping → Origin Address) and “to” address. If one of these addresses have been set incorrectly, carrier will not be able to calculate the shipping cost and find appropriate shipping methods and, as a result, Ecwid will get no shipping methods in response from the carrier site. When shipping methods list is empty, “Sorry, we do not ship to this location” message is shown.
I have checked your Origin address and found out that 90735 zipcode, which you have added there is invalid (http://postcalc.usps.com/Default.aspx
). You need to use the correct zipcode in your origin address at Ecwid control panel → System Settings → Shipping → Carrier-calculated Methods. Once you have set Origin Address properly, shipping methods will be shown as expected.