When you sell a TravelPledge experience, your invoice amount will vary with the size of the winning bid. This guarantees you always generate a meaningful donation.
Access the Donation Calculator for a given experience by clicking the hyperlink in the lower left hand corner of the box. The link you should click says "Keep X% of the winning bid."
Use the arrows to adjust the winning bid. The size of your expected donation will vary as you adjust the winning bid amount.