I have shipped after trees go dormant. I bare root the trees, wash off the roots, and then ship the tree with the root balls completely planted into dampened cocoanut coir, or, dampened milled sphagnum. I then enclose the root ball with a zip lock bag, and tie it to the trunk. I make sure all the roots are covered with the dampened medium, and wrapped tightly. Make sure the zip-loc bag can't open. I cover the actual tree branches and trunk, with dampened newspaper to prevent loss of moisture. Then I ship using USPS Two-Day Priority Mail with tracking. I ship just after the leaves drop off, and while the weather is still cool but not freezing cold.
If you send your trees with the roots wrapped in moss/coir, it will be a lot cheaper than if you ship trees still planted in soil. If you ship trees potted in soil, you need to prevent the soil from falling out of the pots, while in transit. I cut out a thick, Styrofoam disc, and then tape it over the soil so that the soil cannot come out of the pot. You also have to tape, and secure the pot to the inside of the box so that the pot/tree cannot shift around the carton. It's a real pita. I also tie the branches closer to the trunk so that the trees will fit into a smaller shipping carton.
Will your relatives know how, and when to plant a bare-rooted tree? If yes, I'd ship bare-rooted. It's just easier. If the shipping box exceeds certain dimensions, you will pay a premium price for the over-sized cartons. Size matters when it comes to shipping cartons.
Hope this helps. Good luck.
Frank