Okay, but the prebates are not sent only to those making that amount or less. They are for the amount that would be taxed on spending up to that amount for every family. In other words, the sales tax would kick when a family spends over the poverty amount.
From the FairTax website:
"All valid Social Security cardholders who are U.S. residents receive a monthly prebate equivalent to the FairTax paid on essential goods and services, also known as the poverty level expenditures. The prebate is paid in advance, in equal installments each month. The size of the prebate is determined by the Department of Health & Human Services’ poverty level guideline multiplied by the tax rate. This is a well-accepted, long-used poverty-level calculation that includes food, clothing, shelter, transportation, medical care, etc. "