Sorry but can I confirm the size of the parcel is small flat rate box (1-14), medium flat rate box (15-100), and large flat rate box (more than 100)?
According to USPS.com (
https://www.usps.com/ship/priority-mail-flat.htm).
Small Box: 8 11/16" x 5 7/16" x 1 3/4"
Medium Box 1: 11 1/4" x 8 3/4" x 6"
Medium Box 2: 14" x 12" x 3 1/2"
Large Box: 12 1/4" x 12 1/4" x 6"