I was looking at 'Scottish Tartans Authority' website yesterday and found a tartan specific for US Postal Service. I'll try to put up a link. wish me luck.

http://www.tartansauthority.com
Then click on the ferret (I know, it's a little wierd), then click on a-z, then on the letter U. It'll show all the US registered tartans including one that says US Postal Service.

http://www.tartansauthority.com/web/...p?buttonAtoZ=u

I hope the link works. Never tried one before.