Plugin for collecting sponsorship depending on a variable?

I’m hitchhiking to China and would like to fundraise using a ‘sponsor me per lift’ feature.

1. Is there a donation/sponsorship that allows people to sponsor depending on a variable (number of lifts)? I.e. ￡1 per lift.

2. Even better, is there one that allows a cap, i.e. ￡1 per lift up to 100 lifts?

Alternatively, maybe there is a ‘pledge’ plugin that allows me to collect pledges, but not actually donations, of this sort?

Many thanks,
Sam