I was looking, out of curiosity, for the famous Travis Bickle jacket. Then I realized he actually wore two different jackets in the film, a more casual, beige/drab one and a more army-looking, green one. He began wearing the latter in his Training Montage and switched back to the original in the ending. Both have all the same emblems/insignias (apart from the 'We Are the People' pin, which the original lacked). It's never explained, do the Marines issue both and he wore the more savage one as a signal that he wanted to fight the filth off the street?
I figure both of his jackets are from Vietnam, since the more casual one you brought up has BICKLE, T. written on the back. If you're looking for symbolic foreshadowing of his ideas and actions, I figure the mohawk is the clearest one.
However, concerning the items of clothing: the larger army jacket he wears merely enables him to carry that huge Magnum .44 of his around - it probably would have been a bit too visible with the holster had he worn it under the thicker, smaller one. In every scene where he carries a piece, he's wearing the bulkier army jacket. At least that's my take on it.
How did Iris get her money? If you were a pimp, would you let the 12-year-olds in your stable collect their own mail, or give it to them without first opening it?
The film suggests that Sport treats his girls better than the typical pimp. Iris says he's not abusive and doesn't seem to have any signs of abuse. She's allowed to meet clients on her own time (apparently wasn't punished for it). Even when he was being manipulative Sport was gentle and sweet and not threatening. So it might not be beyond the realm of possibilty that Iris would have freedom to read her own mail.
Which makes Travis's rampage all the more disturbing. If he was going to murder pimps at least he should have chosen eviler, abusive ones.
On the other hand, there's no proof Iris got the money. She probably was returned to her parents because the police found her after the murders.