Click on "Apps" on the left menu of your Shopify admin. On the next page, click on "MerchMixer - US Dropshipping".
Click on the "Settings" tab. On the next page, you will find a section where you can add your credit card number, card expiration date and CVV code. Fill in the required information and click on "Update Billing Details".
Done — you've now added your credit card to MerchMixer. Once an order comes in and you request fulfillment, your credit card will be automatically charged the cost of the product.