How do add multiple selections to a related field?

I am looking for a way to add multiple selections relating to a specific field. Our company sells equipment for sports and through the website our customers make their purchases. Now, because the way the site is setup the website distributes payouts/releases funds to our bank weekly. As a result, one payout may be reflecting multiple purchases orders. That's why I was hoping to relate multiple purchase orders to a single payout. I've tried, but I can only seem to link one purchase order to one payout. Any help?