It's a 'voluntary tip'. Explained here:
https://support.gofundme.com/hc/en-gb/articles/115015446027-Extra-Charge-on-Statement
Within the EU websites aren't allowed to have pre-ticked boxes for 'extras' (such as travel insurance when you book flights). If there's a box that's already ticked (effectively tricking you into making an extra payment), you're entitled to receive the additional goods or service free of charge. However GoFundMe is based in the USA, where it's legal to have pre-ticked boxes for things like 'tips', so you have to take great care when using such sites.