I think it can still be done politely, if it's presented as an option.
For instance, the invitation can be "Due to travel constraints, we'll be shipping the gifts back to the couple after the shower. If you prefer to ship your gift to them directly, please feel free to bring a picture or description of the gift to display at the shower. Their shipping address is 123 Sesame St., Anytown, Anystate."
That gives people a heads up that the gifts are going to be shipped back regardless. If they prefer to ship it themselves, they will still be able to present "something," even if it's a picture. If they would rather not go through the trouble of shipping, there's no obligation. However, it would be helpful for guests to know that everything is going to be shipped. They might want to choose something that ships easily, although again, there's no obligation.