To mail overnight to Mexico fold your letter and place in to an envelope, after that lick and seal the envelope, write the name of the recipient of the letter in Mexico, this should be written in the centre of the envelope. Make sure you write the ZIP address of the recipient in Mexico. Also write down your own name and address on the envelope, just in case the delivery fails, you can get back your letter. Affix a 34-cent stamp on the upper right corner. Once all these details have been filled out, courier your mailing package via a good and reliable mailing service. If you mail your package in the morning it is possible for it to reach the recipient in Mexico overnight.