What causes are eligible for tax-deductible donations?

iGive.com is not a qualified tax adviser.  We recommend that you consult a local financial adviser, as each state/country varies in their rules and laws.
If you're donating to an organization that qualfies under section 170 of the IRS code as a nonprofit, your donation may be considered tax-deductible.  If your cause is small, it is possible that it doesn't need to be a "501(c)3".  To ensure that your donations through purchases are tax-deductible, the IRS has defined a set of requirements which are listed here.

Ultimately it is the member's responsibility to know for sure if the cause they choose to support qualifies, or not, for a tax deduction.

For more information, see IRS Private Letter Ruling 9623035.  Remember, ultimately your personal tax situation affects your ability to take any tax deductions, so you should consult your tax advisor.