This is personal to everyone, there is no set way you need to fulfill your orders. If your product is on the cheaper side (costs between $1-$20) you may be okay with ordering your product as soon as an order comes in.

If you want to wait until Shopify pays you, I am including an article from their Help Center that explains the payment days.

If you have a higher ticket store and your product has a large cost, you will probably wait for Shopify to send your funds to you before fulfilling.