When making a donation to a fundraising page a tick box is provided if you would like to top up your donation:
Accepting payments online is a really cost-effective way for charities to receive donations from their supporters. However, there is a cost associated with processing debit / credit card payment, for managing the computer servers that the website runs on and for developing and maintaining the website.
This feature to ask you to ‘cover’ these transaction costs by increasing your donation is intended to be a transparent way of showing you that there is a cost to fundraising, and by covering the cost you are helping the charity you're supporting even more. It is completely optional, but will ensure the full value of the donation goes to the charity.