When a customer submits an order in your connected store, the store receives the retail price (which consists of the production price + profit) and the shipping fee.
After that, the order is automatically imported into Printify and sent to production according to your Order Approval Settings. However, since Printify cannot pull funds directly from your sales channel, Printify charges your linked credit card or your Printify balance with the production price of the product and its shipping fee.
To sum it up: your customers buy from you, and you buy from us.
Read more about setting up payments here.