The statement that is true regarding store credit cards is:
"They usually have interest rates of 5 to 10 percentage points higher than those of ordinary credit cards."
Store credit cards often have higher interest rates compared to general credit cards. The other statements might not be accurate as store credit cards typically can only be used at the specific retailer and are not co-branded with major credit card networks like Visa or MasterCard. Additionally, the grace period for interest may vary, but it's common for many types of credit cards to offer around 21-25 days, rather than exactly 28 days.