American credit cards are used and accepted throughout the world, including in Canada. This is because the credit card providers are large financial institutions on the world market. Although Visa and MasterCard are more widely accepted than American Express and Discover, you shouldn't have many issues with purchasing products while in Canada.
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Call the 800 number listed on the back of the credit card. Many credit cards place a freeze on your account when purchases are made outside of the country (unless you have a travel card or American Express Platinum or Centurion card). This is done for anti-theft purposes.
State the time frame you plan on being in Canada. It is a good idea to call the credit card company a week or two before leaving. (Most don't take information more than a month in advance.)
Look on the storefront window of the Canadian store to see what credit cards are accepted. Like most American stores, the accepted credit cards are usually listed on the front door or window. If they aren't, ask the store clerk.
Swipe your card just as you do in the United States and then sign for your purchase and take your goods with you.