regarding your sponsorship demo, i would expect the amount that i enter to be the total that i'll pay, because it is determined by the money that i have. the fees should not be added but subtracted from that amount. they also should be shown on the same page as the split to have a complete overview of where the money goes, not hidden as a surprise added on checkout. (hiding fees is actually illegal in the EU. i am not sure if this qualifies, but i'd be careful)

btw: consider a monthly (or other interval) repeat option