Costco is usually a very good way to go for any policy 14 days or over, especially if you go with the Limited Territory option (Mexican border states). Unfortunately, it doesn't look like Costco will be providing Mexican auto insurance much longer. Who knows though, maybe they will be going with another provider instead of their current one, Bonita West.
FYI: For those of you who currently have a policy with this Costco provider, here is what Costco is saying about it:
As of midnight, Saturday July 18, 2015 Bonita West Mexico Insurance Services will no longer be selling discounted insurance for Mexico in partnership with Costco Warehouse. All insurance certificates sold up to that point will be fully honored and serviced by Bonita West and the Mexican underwriters Mapfre Tepeyac and Qualitas until the expiration date. Please call Bonita West customer service at 800-265-1365 if you have any questions. We would like to thank all Costco members for their business and their loyalty and wish you all safe travels.