Use online shop (store.origin.com) and close Origin. It will redirect you to the mexican store. Easiest will be if you use Chrome (it translates the spanish into english) with the plugin foxyproxy, which will allow you to setup the proxy.
You disconnect it at the point where it asks you to login to your account (after you click on the checkout button). Then you log in with your Origin ID and pwd and just follow the normal process.