Try:
1. Check your delivery process. If your packet has not been delivered to USCIS yet (i.e., the delivery is still on the way), then contact the delivery company such as UPS to cancel the delivery; or
2. Cancel the payment checks as you plan and send a letter to USCIS to withdraw your applications at the same time. But you need to realize that if USCIS have processed your case, they will ask you to pay the fee even if you cancel the payment check. If USCIS have not started processing your cases yet, then you might be able to get your money back.
