We sent the original copy. You can make an copy of the I-94. If you cannot get approval notice before the expiration date, your parents can submit the copy of I-94 at the airport. I heard that some people submitted copy of I-94 and their application was approved. But I also saw cases that were denied since the materials did not follow USCIS instruction. If you want to minimize the risk, it is better to follow USCIS instruction. Good luck!