You should never use your debit card for online purchases. That is extremely risky.
People's online information get hacked every-single-day. Eventually - and likely - you will also.
And that hacker has access to your real money.
On a credit card, you are guaranteed not to be held responsible for fraudulent charges.
Debit cards are also protected, but many banks require, by law, no more than 48 hour protection. In other words, if you didn't notice a charge you didn't make two days after it occurred - you are screwed.