Can you consider shipping the tiers separated and giving her detailed instructions on how to stack them? That would be the only way I would do it!! Shipping with dry ice is incredibly tricky as well, something you would need to discuss with your local post office as they have to be aware that dry ice is in the package. I also would never consider shipping cakes unless the client knows the risks involved and that any damage is not your fault and will not be refunded.
-- Nikki, So Cal, www.Facebook.com/nikkibelleperchecakes