They call them Go Packs, and this is the way they work: Boston travelers can buy 10 one-way flights between Logan International Airport and all Washington, D.C.-area airports — Washington-Reagan, Washington-Dulles, Baltimore — for $699 plus $7 in taxes and fees per flight, or about $77 per trip. You can buy the Go Packs through April 17 for travel from April 23 through June 27. One useful feature: The Go Pack can be shared. When you buy one, you get 10 separate codes, one for each flight you've prepaid. If you want to share with a friend, you can just let him or her use one of your codes for a one-way flight. If you do share, though, be aware that you must set up your tickets on separate reservations. Here are all the details.