Is the aforementioned commercial transaction, which involves selling print-on-demand products online, considered a Salam contract, an Istisna'a contract, or selling what you do not possess?

This transaction involves two contracts: a Salam contract between the inquirer and the customer, and an Istisna' contract between the inquirer and the company. The problem lies with the Salam contract, because it requires full payment of the price at the contracting session, which does not happen here since the intermediary website is the one that receives the payment and does not transfer it to the inquirer until the sale process is completed.