The telltale postmark (spoiler)
When the polaroid is mailed at the end of the movie, the two stamps are from Mexico. Right? But look at the cancellation marks. They are goofy.
The top stamp has only wavy lines, and the bottom stamp has a "quarter-circle" cancellation (a real one would have a full circle) and none of the cancellation ink is on the envelope. Deduction: Some cancelled stamps were just pasted on an envelope.
So how did it get to the detective? The post office would not deliver it like that! They don't let folks pull foreign stamps off old letters, re-stick them on mail and resend.....
Was that part of the plot, or goof from the film production folks?
"When you throw dirt, you lose ground" --old proverbshare