Unfortunately, the United States does not have 'Interac' in the way that we think of it here in Canada. Canadians are the #1 users of debit cards in the world. It's practically unheard of to be using debit cards to buy 'regular' items (groceries, food, etc) in the United States.
When an American says 'debit card', they really mean 'credit card' connected to a bank's debit system (much like the PayPal 'debit card' is really a MasterCard connected to your PayPal account.) So, if you can accept credit cards, you can accept American 'debit cards.'
-RS.