What should one do if they bought goods without paying their full price due to a mistake by the seller, and then the shop closed, making it impossible to ascertain the remaining amount due in order to pay it?

You must inquire about the company and its owners. If you find them, return their rights to them. Otherwise, if the owners are Muslims, give it away in charity on their behalf. If they are non-Muslims, you may benefit from it, and return it to them if they appear at a later time.