Is it obligatory to return the commission received for selling a pet that died three days after reaching the buyer, knowing that the cause of death was bladder stones, and that the seller fulfilled all obligations including a veterinary examination and informing the buyer about the animal's genetic traits?

If the animal is permissible to sell and own, and you agreed with the buyer on a commission for shipping and tracking it, then there is no blame upon you for benefiting from the commission, and you are not obligated to return it due to the animal's demise, because you are not a guarantor of its survival, nor did you neglect its shipment.

However, if the animal is impermissible to sell, then the brokerage contract for it is unlawful, and what was taken for it is also unlawful. But it should not be returned to its payer; rather, it should be given as charity to the poor and needy.