What is the ruling on working in dropshipping, knowing that I act as an intermediary between the supplier and the customer, and I purchase the product from the supplier and ship it directly to the customer after the customer orders it and agrees to the store's terms which clarify that I am an intermediary?

Based on your description of your work as an intermediary between the supplier and the customer, and your explanation to the customer that you are an intermediary, this is considered brokerage (samsara), and it is permissible to take a commission for it from one or both parties to the contract. It falls under the category of ju'alah (a promised reward for performing a task). The one paying the commission must be aware of it, but the other party is not required to be aware of it. The fact that the commission is the amount in excess of the specified price does not affect its permissibility.