What I've done with a 100% success rate is to use a #10 envelope with a pre-printed return address label and computer printed label for the DX station's address. As already mentioned I put no hint of ham call signs on either label .... this makes it look like business or marketing mail. I enclose a #9 envelope with my pre-printed address label on it, put that, my QSL card, and a few dollar bills inside a folded blank piece of paper. I also keep a supply of USPS forever international stamps on hand. I don't mind one bit sending a few dollars to help cover the cost of postage etc. for a card from a new country, and will gladly mail a card to a DX station that needs my county, state, or whatever.