The issue is on the Additional Rates on shipping flat rate method.
I added some additional rates as it suppose to be, and its display well, but i get "invalid shipping method" when i clicking on the button to continue the process of buying.
the way to add additional rate is:
description | price | per cost type (order...)
when i am doing it on English, it works perfect, but other languages like Hebrew - i got error of invalid shipping method.
plugin Version 2.0.20
i cant update the Woocommerce plugin because someone did customization to the site on woo files.
I did it finally by this: http://docs.woothemes.com/document/shipping-method-api/
This doc helped me and now it works like a charm.