Having a merch person who is friendly and really sells what you got can help keep your wallet thick and your group out on the road. The difference between a lazy merch person and a proactive one can reap merch sold, which leads to free advertising and new fans as other people hear your group. If the merch person is proactive getting people to sign to your mailing list it can be the difference between increased show turnouts and diminishing returns. Giving the person selling your merch a percentage of what is sold can keep them being proactive and get them to make sure they do a good job every night of the tour. Give an incentive to perform better and you will see a better job done for a repetitive and boring job.