Citifinancial Retail Services back billed interest by posting payment past promotional expiration date Tempe Arizona

I purchased a mattress on a 9 month no payment no interest promotion. Instead of waiting until last month of promo to pay off balance I paid monthly payments to lower the final payment when it was due. All 8 of those payments were posted to my account within 3 to 4 days of my mailing the payment. I purposely mailed the final balance 15 days before promotion expired so there was plenty of time for payment to be recieved and posted to my account so I could avoid paying any interest charges per the promotional plan. My promotiomal plan had an expiration date of 4/25/2008 I mailed my payment on 4/10/2008. The statement and customer service person says they did not recieve payment for the entire balance until 4/26/2008 and posted it to my account on 4/27/2008. How is it that 8 payments arrive and are posted to my account within a 3 or 4 day period of my mailing them and one does not? These people are ripping me off to the tune of $126.25 for back billed interest. How many other people are being ripped off by this scam? All payments we mailed to Cloumbus, Ohio from Michigan.

Ernest
Gaylord, Michigan
U.S.A.