Charities rarely can guarantee donors will be satisfied, but some groups are taking extra pains to ensure contributors are happy.

Among them: DonorsChoose, a group that lets donors pay for education supplies and other needs listed by schoolteachers on a Web site.

In exchange for contributions to a teacher of their choice, donors are promised a statement from the teacher describing the difference made by the gift, thank-you letters from students, and photographs of students putting a donor’s money to work.

But, about 2 percent of donors never get those materials. So now those donors get an e-mail message offering them the chance to pick another teacher to support, and DonorsChoose covers the cost. To pay for the extra donations, DonorsChoose is using a $30,000 grant it received from a donor who wanted to help the charity back up its promises.

Donors who receive the DonorsChoose offer are delighted and surprised, says Charles Best, the founder of the charity. “Proactively admitting a screw-up generates a whole lot of good will,” he adds. “Instances abound of donors saying they will give more” after receiving the offer.

A similar offer was recently created by another online-giving group, GlobalGiving, which began offering GlobalGiving Guaranteed few months ago. It matches any gift from a dissatisfied donor up to $10,000.

What ways have you found to keep dissatisfied donors from giving up on your cause?