I've worked at businesses that employed salesmen. The salesmen opens the door for the business. Had it not been for the salesmen, the business would not have had access to the customers the salesmen generated.
I understand the concern. Why pay the salesmen for a customer he hasn't maintained since the sale? It would be nice if the salesman helped to maintain the account, but all he's really doing is selling YOUR product. It's your job to keep the customer.
Everytime the customer orders your product, remember it's the saleman who generated that sale, even if he's done nothing since. Cut the salesman his check and send your customer the product. You wouldn't have the sale without that person's effort.
You'll never be able to prove the customer would not have been able to find you without the salesmen, but we all live in the real world. Chances are pretty darn good that it was the salesman who got you the customer. Remember, there are thousands of us that make tshirts.
It's money well spent if the person you hire to generate sales actually does so. Give the salesman the benefit of the doubt and cut the check. If you don't, next time a sale presents itself, he may not mention your name, he may mention someone else. Now where will your sale go.