mine is flat in many sections. That is post - meeting my hammer on the garage floor. You were looking at the same stuff as what I used - $1.60 at Lowes here.
Flattening it slightly with the right technique results in bending the pipe, and you can flatten it. Not that you would want to take all the time to flatten a whole piece; it wouldn't be perfectly straight anymore. Sorry to get you all excited.
Just curious, why do you need it to be completely flat? The tubes are as strong as a bar 2/3 their size. (just in case you didn't know - you may already)
How an I-beam works