You can create orders and send invoices directly from your Shopify admin. This lets you handle purchases in a few different ways:
- create new orders and enter credit card payments in Shopify for sales you've made by phone, in person, or elsewhere
- send invoices to customers to pay with a secure checkout link
- use custom items to represent additional costs or products that aren't displayed in your inventory
- sell products at discount or wholesale rates
- take pre-orders
Check out the full documentation over on the Shopify manual.
https://docs.shopify.com/manual/your-store/orders/create-orders