Most credit card companies in the US do not charge any interest on any purchases if you pay at least the statement balance every month. E.g. you effectively get between 25 and 30+25 days of interest free grace period, depending on where in the billing cycle the purchase was made.
For example, this is what "Quicksilver From Capital One" currently says under #disclosures:
Your due date is at least 25 days after the close of each billing cycle. We will not charge you interest on new purchases, provided you have paid your previous balance in full by the due date each month. We will begin charging interest on cash advances on the transaction date.
However, what happens if you buy something in one statement, but then return it the next one, and the refund posts before the payment due date for the prior statement?
Do you still have to pay full statement balance of the previous statement to avoid being charged interest, even if some purchases have been refunded prior to the payment being due?
What about other account credits or adjustments?