So then who decides what's reasonable? Take this Knize split for instance. I had no idea of the popularity that it would draw. So now, instead of doling out one split and all that entails, I'm going to be doing 2. Or maybe 3 even at this rate. So how do I factor in all that time and effort? If I say to everybody "OK, there's going to be a $5 surcharge tacked on top of the shipping charges." everybody would fucking riot. People tend not to like it when you spring extra fees on them.
I think as the size of the split grows, the surcharge should decrease.
If you charged $5 for each person in your knize split with 24 spots, that gives you $120 for your time. I think a $2-$3 surcharge for this size of a split would be reasonable.
If there was a split for, say, 5 people, then maybe a $5 surcharge would make sense.