You will need to make sure they are a qualified organization- it depends if they are a U.S. charity or not. You would not need the receipt from the charity, unless there were any single donations of $250 or more as long as you can substantiate the donation.
Under the U.S.–Mexico income tax treaty, a contribution to a Mexican charitable organization may be deductible, but only if and to the extent the contribution would have been treated as a charitable contribution to a public charity created or organized under U.S. law. To deduct your contribution to a Mexican charity, you must have income from sources in Mexico.
Donors are responsible for obtaining a written acknowledgment from a charity for any single contribution of $250 or more before the donors can claim a charitable contribution on their federal income tax returns.
A donor may not claim a deduction for any contribution of cash, a check, or other monetary gift made on or after Jan. 1, 2007, unless the donor maintains a record of the contribution in the form of either a bank record (such as a cancelled check) or a written communication from the charity (such as a receipt or a letter) showing the name of the charity, the date of the contribution, and the amount of the contribution.