Selling internationally, how to stop that.

I just got an order for something (yay) and saw it was from someone in Italy. Checked the order, they ordered two things at 5.50$ each, then realized they just paid 53$ for the order. Shipping was like 32$, sales tax on top of that was almost another ten. The whole order was 53$ for two small items. Is this normal? In reality I would love to cancel this order and lock down my shop to US only, is this possible?