You don't need a PIN to use a debit card online.
Yes, in some places you do, like Canada, where most debit (bank card) transactions are not handled on the traditional payment processing networks. You actually need to login to your bank account to pay with Interac online.
That is slowly changing. A few credit unions and one of the big banks have introduced Debit MC/Visa cards.
Interesting that you don't in the US.
In the US the vast majority of debit transactions are processed over the Visa/MC network. The debit card you get from a US bank has a 16 digit number, and MC/Visa logo, and for all intents and purposes is a Visa/MC card, and can be used for card-not-present transactions just like any other Visa/MC card. Just because a card has a Visa or a MasterCard logo doesn't mean it's necessarily a credit card. It could be a debit card, a prepaid card, a gift card, a corporate expense management program card, etc. That is the case worldwide.
IMHO, in Canada Interac will be going the way of the dodo here as well - CIBC has already introduced hybrid Interac/Visa debit cards, and frankly, Visa/MC debit are far more useful to consumers as they have nearly universal acceptance. Merchants hate them though, because they pay the prevailing MC/Visa fee instead of the Interac fee, and the banks (which own Interac) hate them because they have to cut Visa/MC in for a percentage.
As a cross-border merchant with warehouses in both US and Canada, I do accept US debit cards (because they're just regular Visa/MC cards to me), but not Interac, because you have to bounce to a third party site to authenticate the transaction.
In reality, no one should be using their debit card for online transactions, because the money is immediately gone and you have to get it back in the case of fraud as opposed to a credit card where you simply don't pay the fraudulent charges.
I require a pin number for orders.
Unless I'm mistaken, you cannot ask for a PIN number for card-not-present Visa/MC orders. Check your merchant agreement.
Note I'm talking about an actual PIN, not a CVV code.