I suggest that you set up an two purchase options for the model A & B. The first option is your base price for your device (i.e. model A=$25). The second option is the device with $5 donation (i.e. model A ~ $30). This second option would allow for easy donations and avoid a second transaction fee for credit cards & paypal. Larger donations would be done by a normal dedication donations page that takes credit cards/paypal.
Another option would be a "buy one + give one" option like the EEEpc did.
I know that R-PI is a charity, and working a little "profit" into the device as an option can help fund future work and allow for the charity to donate devices to those who need them.