All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
As for buying goods and authorizing the website to store them and then pack them and send them to your customers (on your behalf) in return for a commission (fee) that you pay to the site, then there is nothing wrong with this, and it is permissible to deal with the mentioned site in what is permissible.
In all transactions, you must abide by the conditions of the sale, whether the sale is done directly or through websites or other means; those conditions are as follows: the goods must be lawful, pure, beneficial, in the possession of the seller, and deliverable to the buyer; the goods must be known (to the buyer) by seeing them or by describing them completely (to the buyer) by specifying their amount and type as well as other matters which make the product known to him.
Allah knows best.