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.
I used the priority shoebox sending a 1 lb. 6 oz. item from San Diego to Boston and the charge was $11.25. This was 2 day service. I would think if your item fits in the medium size box and weighs 3 lbs or over, the flat rate priority box would be less expensive than the shoebox. I think flat rate priority medium size is about $12 or $13.