Re: Made the cardinal mistake.
... after you've removed all evidence that you were ever there or ever touched the envelope
1) Be aware that a modern printer will be likely to tag all of the pages with an ID code (http://www.seeingyellow.com/.
2) You will probably never get all of the DNA off the envelope or paper; it might be better to dip both in the contents of a vacuum cleaner bag aquired from a pup dumpster.
3) Never give "authority" a break, 'do unto them before they do you' is the rule of the land.