Could be stress related, or shipping related. its not ‘red leg’, atypical location, and the way it looks. You should contact the breeder, if it is reputable as you say, she/he should refund at least and might have better idea, since she/he saw a frog and packed it. Normally they die so soon after shipping because of preexsisting conditions, improper packing, stress normally a factor too, but it is alone shouldnt be high enough to kill the frog. My question is... shipping in that kind of weather... and in general in winter. Myself i do not ship animals in winter at all, too risky.