Social Security Disability back pay will be paid in a single lump sum and will be reduced by the amount of SSI already paid for months when eligibility for the two programs overlap. The second SSI installment is due this month, but sometimes the installments are delayed.
The medium flat-rate boxes for Priority service are 13 5/8 inches by 11 7/8 inches by 3 3/8 inches and 11 inches by 8 1/2 inches by 5 1/2 inches. Shipping charges are $12.65. There is one small-size box offered for Priority service. It measures 8 5/8 inches by 5 3/8 inches by 1 5/8 inches and costs $5.95 to ship.
This is the link on USPS site that explains maximum size for a flat and if its bigger than its a parcel/package. 3/4 is the minimum for package. However, I have sent items in bubble mailers that weren't quite that thick and have never had a problem. http://pe.usps.com/businessmail101/mailcharacteristics/flats.htm.