“We’d like, but don’t insist on a donation.”
The actual amount is anchoring - by specifying an amount on the high side, they’re resetting your expectation for what the amount should be.
It’s an easy psych experiment to do: set up two small identical events worth a couple bucks of people’s time. For one, suggest a donation, but not an amount. For the other, suggest a $20 donation. The second case will fairly reliably get higher donations across the board, even for people who don’t give the whole $20.
It’s exactly the same principle as the the absurdly high-priced “premium” model of something in shops: $10,000 throw rugs and the like. They don’t expect you to buy it, but it produces a dramatically increased number of sales of the “mid-range” $500 one as opposed to the “bargain” $100 one just by being there.