But the scientists say that only about 10 % of reading time is spent moving the eyes, and the inability to reread previous sentences when using rapid serial visual presentation will result in a failure to understand the text. (goodereader.com)
Many providers charge a monthly fee of $ 1 to $ 4 if you want to invest the balance (small, but it adds up, especially given the low balance per previous sentence). (money.stackexchange.com)