IMHO, no this isn't a way to treat a customer. Even if it's a 4.95 customer, they should still be treated with respect and professionalism by you....the customer support dude.
Even when it's hard and they attack you and you just want to verbally castrate them...YOU need to be the adult and the businessman and handle things professionally. Sometimes it means biting your tongue (typing fingers) but the customers are you're bread and butter and NEVER bite the hand that feeds you.