What about just providing the GOH's shipping address with a note that says something like, "If you'd like to ship a gift directly to Jack and Jill, their mailing address is: ________ " or even possibly "Since Jack and Jill will be traveling to the shower, if you'd like to ship something directly to them their mailing address is: ________"
That leaves it open for guests to gift what they want, but also reminds them that the couple will be traveling and will need to transport gifts.* Plus, it gives people who want to ship the address. I find that I often don't have addresses for out of town friends because I correspond mostly over phone and email and many of them move around somewhat frequently. I also prefer not to gift something that might be a burden, so I'd want to either get them something small in size or buy something and ship. So I'd consider having the address right there really convenient.
*More than once I've casually mentioned something about shipping vs. bringing a gift for the recipient's convenience to a third party and have had them remark about what a great idea it was and that they'd never considered that. So I'm not sure it is necessarily something that occurs to people unless they know someone who has had difficulty transporting gifts.