An airmail letter or postcard weighing up to 20g costs M$11.50 to send to the US or Canada, M$13.50 to Europe or South America and M$15 to the rest of the world.
Delivery takes 15 to 20 days, or five to eight days if you pay for express service. (Or occasionally never, all part of the fun.)
If you’re sending a package internationally from Mexico, be prepared to open it for inspection at the post office; take packing materials with you and don’t seal it till you get there.
For assured and speedy delivery, you can use one of the more expensive international courier services, such as UPS (www.ups.com/mx/es) or FedEx. Prices vary due to exchange rates and what you are trying to send.