If you are leaving July 21, there is very little chance you could get your actual cards by your trip. While I've read one report that someone got their cards in 6 weeks, most people get them at 8 weeks and some people get them even later than that and you only have 6 weeks until your trip.
So I think you will have to be content with using the letter for this trip. If you order at least 2 weeks before your trip, they will mail your confirmation letter. If you order within 2 weeks of your trip they will leave your letter at Epcot for you to pick up.
Be aware that the letter has its own expiration date so if you order the DDE now your letter would probably expire before your trip. So if you do decide to order this week, ask them to put an expiration date on your letter that is later than the last day of your trip.
If you want the latest possible expiration date, wait until July 5th or 6th to order the DDE. That still gives the DDE office enough time to mail your letter and since that will be the second week of July, hopefully they will have bumped the DDE membership expiration date from July 31, 2006 to Aug 31, 2006.