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Most companies will post their cancellation policy or a link to it on the order page. As a result, before you cancel, you should read the cancellation policy. Some companies require you to cancel an order within 30 minutes or an hour of placing it. Some companies may allow you 24 hours to cancel an order. Many companies will not allow you to cancel an order after it has shipped. The company's cancellation policy may stipulate that you must pay a cancellation fee. This fee is usually a flat rate, although it may be a percentage of the overall sale. In the case that a product has already shipped, you may need to pay a fee and shipping cost of the product. If you used a credit card to make the purchase, you may simply be able to call them and cancel the purchase. Ultimately, this depends on the specific credit card company and their policies. For instance: American Express has a very customer-oriented policy and will typically cancel a purchase at their cardholder's request. Most Visa, MasterCard, and Discover cards may not cancel a purchase unless it was somehow fraudulent.
Read the company's cancellation policy. Pay a cancelation fee. Talk to your credit card company.