Hello over there
When a customer places an order in your Shopify store you receive money to your payment gateway account (PayPal for example).
You can find available payment gateways by the country here. https://www.shopify.com/payment-gateways
Then, you have to use your personal credit card and purchase an item for your customer from the supplier, to be shipped directly to your customer.
There is a certain authorisation period for money to be transferred from PayPal to your account. That is why you'll need a small amount of money in your personal account for your initial orders.
I'm afraid there's no way to use that money your customer paid just right away.
But not to worry The money will still reach your account sooner or later and you will then be able to use it for further purchases!