What is the legitimate period to await the seller's response before disposing of an extra mattress he sent by mistake, and is it permissible for me to use it or must I donate it?

If the buyer receives an extra item beyond what he purchased, it is considered a trust (amanah) in his possession and never enters his ownership without the seller's permission. If it perishes without misuse or negligence, he is not liable for it. A consequence of it being a trust is that the cost of returning it is borne by the seller. If the buyer despairs of returning it to the seller, he may give it away in charity on the seller's behalf.