Paying online with a debit card is safe.
As long as you're on a legit website that has the lock Icon in the address bar and has the https: then the website is secure.
When you use your debit card or credit card online the website owner does not see your debit card information or credit card information.
Your credit card or debit card is processed by a third party credit card processor.
When you pay for something online with your debit card or credit card the payment goes through a payment processor.
Then the payment processor eventually releases your payment to the company you paid the money too within a week.