You need to understand that you'll get a % of returns and to be honest, the margins are so high from dropshipping, you are better off just giving the person a refund and letting them keep the item. Another route is to have them send it back to the factory and they have to pay shipping, but you'll get a negative customer than, all over a $20 item.
And if the quality becomes an issue, then you need to quickly adjust your suppliers to make sure it doesn't keep happening.
RE: How To Make Money With Dropshipping on Shopify