Selling something you do not possess is not permissible.

I am setting up a subscription business insha’Allah For the first 3 weeks of every month, I will gather subscribers, who will sign up to get a box of goods. During the last week of the month I will gather the box of goods, and then send them out I will NOT bill customers until the start of next month, ie, until after I take possession of goods. Permissible?