Cut pencil width or a little thicker cuttings with new buds on them. That means with a leaf on the branch or recently had a leaf since the buds are right under the leaves.
Wrap them in parafilm if there's not too many. Seal them inside a plastic zip bag, with moist paper towel if not wrapped in parafilm. Wet the paper towel and squeeze until it doesn't drip.
In the US, use first class mail to save on postage up to something like 3 ounces. Then priority flat rate small box is better.
For international, there used to be parcel post but they got rid of it to make more money. Try not to use priority since that's more expensive. If you ship something like a medium priority, postage alone is something like $60. Smaller packages like scions are worthwhile if you ship enough scions.