A standard wedding gift when I can't attend due to distance is to give a gift certificate for Harbor Fish, here in Portland, Maine. The lobsters are Fedex'd and everyone has been very pleased with their service.
Two other markets that I frequent and are definitely reputable are below. Additionally, you can usually have more than the lobsters shipped. Most places will ship bibs, nut crackers, etc. Just ask. All these guys are very good as are most places in Maine. These are just the ones I usually deal with as I can walk to them. Prices are pretty low this year although for shipping purposes, you'll want the more expensive hard shells. Enjoy!