Debit cards are different than credit cards. A credit card is a form of unsecured credit, meaning the money you spend on a credit card doesn't actually exist until you pay the bill at the end of the month. But a debit card is linked directly to your bank account, and transactions made on your debit card come straight out of your current account. Many institutions agree that using a debit card online is risky, and it is important to notify your bank immediately if you or someone else has made an online debit charge you need cancelled.
Attempt to resolve the problem with the merchant. You will need to return any products you bought and you should offer to provide a copy of your receipt (keep the original receipt for yourself). Call or e-mail the merchant directly and explain that you would like to cancel your transaction and get a refund.
Check the bank account that is connected to your debit card. If the merchant did not agree to give you a refund, money will be taken out of your account. Once the charge goes through and is no longer "pending," you can dispute the charge.
Contact your bank. You now have to dispute the charge just like you would for a credit card. Fill out any dispute paperwork your bank requires and allow it to investigate the case. The Federal Trade Commission recommends also sending a copy of your receipt and a letter detailing the dispute to your bank.
Follow up with your bank if you do not receive a written answer within 30 days of your dispute. The bank should provide you with a free copy of its investigation as well. Some companies such as Chase may even try to speed up the dispute investigation by speaking to the merchant and you at the same time in a conference call.
Fight the merchant in court if necessary. Unfortunately, debit cards are not protected under the Fair Credit Reporting Act (FCRA) like credit cards are. Visa and Mastercard both have zero liability programs for debit cards with their logo, but the rules are strict and you may get stuck paying some or all of the charge. If you have the receipt and the merchandise purchased, consult with a lawyer.