Sending DNA kit from Australia to Ireland

I tried to send my kit via AusPost from the post office and they charged me and made me fill out a customs form. The kit was returned by IRE customs. I have no idea why it was returned though.

Ancestry have given me another kit but I don't want the same problem again.

Has anyone had any luck with dropping it in street post box with the Ancestry pre-filled form on the box?

Thanks if you have any suggestions.