It’s easy to document any cash/check/charge donations to charities collecting for major disaster relief. All major agencies will send or give a receipt for your donation. It’s important to keep it for your tax records.
If you donation items such as food or personal products to a disaster relief effort (many major organizations collect products and truck them into disaster sites), you need to keep the receipt for the purchase of those items. Write on the receipt the name of the organization to whom you are making the donation. In addition, if the receipt contains items you are NOT donating, circle the items you are donating and total those items on the receipt. Keep the receipt for your tax records along with any receipt you may receive when you drop off your donation.