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 $5.95 to ship.
Probably close enough for the Post Office calculator. So using 25x3x5 (the 3 came from 6 x 1/2) as your package dimensions, postage will be calculated just above the small flat rate box ($.05 in my test case for a 1 lb. package). I used the Large Package option because it allows you to input dimensions.