One of my Favorite US Mint Collectible is the First Day Covers.  Numismatic Guaranty Corporation (NGC) is a Major Grading Company who developed a semi-rigid plastic sleeve for housing the entire first day cover with coins from Philadelphia and Denver(for the Presidential Dollar and State Quarters).  You can see the Coins, the envelope and stamp cancelled on the the First Day of Issue and Read History about the Coins.  I have had many Covers graded and auctioned many through Ebay.  They seem to be getting more popular with collectors, I think they're great and can be enjoyed and handled in the Graded Sleeve.  Note: One Drawback is Coins Don't fall under NGC Guarantee like coins put into their slab.  I think keeping the Cover together is worth it versus removing them and putting them in a traditional sleeve, let me know how you feel, thanks again to all, shockmeddy