Make a purchase at a major retail chain, and you might be asked if you would like to sign up for that chain’s credit card. You might be promised appealing perks such as a big discount on your first purchase with the card and ongoing discounts. But be warned—these store-branded cards often have extremely steep interest rates.

The average interest rate on credit cards from the largest US retailers is 23.84%, compared with an average of 15.42% for general-purpose credit cards. The highest rates are for store-only cards from Big Lots (29.99%)…Zales Jewelers (29.24%)…and Staples (28.24%). Using store cards such as these could be a surprisingly costly mistake if you cannot pay off your balance in full each month.

Example: If you make a $500 purchase on a store card that charges a 24% interest rate and then pay off $25 each month, you’ll pay $145 in interest—nearly one-third of the purchase price.

But there are store cards with more reasonable interest rates. These include Dillard’s American Express (10.24%) and Nordstrom Visa Signature (11.15%). These low rates are based on creditworthiness, however, and aren’t extended to all applicants. (For a list of interest rates on 68 store cards, type “2016 retail survey” into any search engine.)

Some store cards offer such hefty discounts on cardholders’ initial purchases that this alone can make it worth signing up—assuming that you’re making a large initial purchase and that you will pay the bill in full.

Examples: Belk Rewards Card recently offered 20% off on select first-day purchases…My Best Buy Visa Card (from Best Buy), 10% back on the first purchase…JCPenney Credit Card, 15% off the day the account is opened (but only 5% off major appliances)…Toys “R” Us Credit Card and Toys “R” Us MasterCard, 15% back on the first purchase…Kohl’s Charge Card, 25% off the first purchase.