We need a card on file because we do not have access to the funds from your Shopify account. Your customer pays you. Then you pay us the wholesale cost of the coffee.
So to pay for the wholesale cost of the coffee you sold, we have to charge a card you have on file with us.

When do I get the money from my Shopify account? 
It depends on where you are located. Click on the link below for more details on how you get paid.
How you get paid

When do you charge my card?
We charge your card immediately after the order has been placed. If the charge is successful, we begin processing your order.
If the charge is not successful, we send you an email to notify you. You must pay for the wholesale cost of the order before we begin processing the order.
If the charge is unsuccessful, you can go into the Dripshipper app and click on the "More" button on the top menu and then "Invoices". Find your unpaid invoice and click the "Pay" button to pay for that invoice.