Pronunciation differences between θ and �!

You are at the school store buying sweatshirts for you and a couple of your friends.  The person behind the counter asks you how many sweatshirts you would like.  You answer her by saying, "I would like three sweatshirts".  She says "I am sorry but you have to pay for them" and you say, "No, I want three sweatshirts".  She repeats "You need to pay for your sweatshirts we can't give it to you for free".  You get discouraged and leave.  What happened here?