That's a common misconception. Quite often, if you owe the WS it means you MADE money. It just depends what do you owe them for?
If I send 100 orders at $60 ea that's $6,000 outgoing, of which I have collected $6,000 + $5 service charge / order = $6,500 in my pocket right now. At the end of the next month I have to pay the WS 80% of that $6,000 = $4,800. On those 100 orders I also earned a $4 rebate, reducing my debt to the WS to $4,400.
At first glance ... Oh, my! I owe them $4,400!! The @#$%@ must be ripping me off!
Actually, I just profited $1,100 (that's $11 per order profit, less marketing, sales & admin costs), even though I'm paying the WS.
Now, if the debt is for ASB listings, failure to REJ, or other useless nickel & dime fees, that's a bigger problem.
Flip side is if the WS is sending me a cheque that means I filled a lot of orders ...