I got the same RFE. What they asked for is: tran or I-20 to prove full-time student status. I did what they asked for and was told it everything is O.K. 
Just do what they ask for. I-20 or tran should be enough. Of couse you can ask your ISSS to provide a letter. 
Don't worry about that. A lot of time, they just don't look at your materials carefully. 
回複:I485 RFE for F-1 Student 懇請建議
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                    回複:回複:I485 RFE for F-1 Student 懇請建議
                     -xmmm- 
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                        03/13/2007 postreply
                                                12:03:36
                    
                    
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                    回複:回複:回複:I485 RFE for F-1 Student 懇請建議
                     -lovewind- 
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                        03/13/2007 postreply
                                                12:22:22
                    
                    
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                    Thank you!
                     -xmmm- 
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                        03/13/2007 postreply
                                                12:28:32
                    
                    
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                    回複:回複:I485 RFE for F-1 Student 懇請建議
                     -xmmm- 
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                        03/13/2007 postreply
                                                12:10:11