Upvote:4
Since you have a valid US visa, you do not need a Mexican visa or electronic authorization to visit Mexico.
Starting May 1st, 2010, all foreigners, regardless of their nationality, visiting Mexico as tourists, transmigrate or for business matters will not need a visa for Mexico as long as they hold a valid US visa and arrive in Mexico from the United States.
Upvote:13
According to this page: http://visasmex.com/files/SAE_eng.pdf an electronic authorisation is valid for 30 days from date of issue. Hence it seems you cannot apply for this authorisation more than 30 days in advance.