A: On-line services like thebigday.com and thehoneymoon.com allow couples to register for goodies like guided tours and portions of airfare or accommodations. A few caveats: Sign-up can involve hefty service charges or a flat cost (up to $150), and buyers may incur purchase fees of 6 to 15 percent. Also, if your guests haven't paid the full cost of your plane tickets by the purchase cutoff date, you'll have to pony up the difference. (You'll be reimbursed by the registry for any additional contributions.) Still, it's a small price to pay to get Aunt Betty thinking "London" instead of "lawn ornament."