Microsoft's Bing Rewards program has been going on for a number of years, allowing users to collect points when they use the Bing search service on their PCs and smartphones that can be exchanged for gift cards or, in this case, donated to charity. The Surface donate page mentions the ending of this particular option:
Until June 30, you can continue to donate credits to help your school meet its goal. After June 30, schools that met the 30,000 credit goal will be sent their free Surface bundles. Donations to schools that haven't met the 30,000 credit goal will be credited back to the donors.
The notice adds that Bing Rewards members will still be able to donate their points for other charitable causes like Teach for America and the Boys and Girls Club of America.
Thanks to "nicholaslovan" for the tip!