When requesting a donation, is it better to have a prefilled button or enter their own value? [duplicate]

A non-profit organization is selling tickets to an event. I suggested they add an opportunity to make a tax-deductible donation line along with the ticket purchase, like:

2 Tickets .... $45

Make a donation!

Your tax deductible donation blah blah

Suggested donation

[$10] [$20] [$50]

They liked the idea but insist that the buyer fill in a dollar amount instead of clicking the suggested amounts cause

Its not about what is easy. Its about stewardship of relationships and this is not the place to make a formal "ask". We're simply offering the opportunity in case they have the money or the inclination to donate to us.

I don't understand their point. From a "sales" point of view, I would think being able to just click a button is more enticing while entering a value is just more work on the user's part.