- You can make a Customer Purchase and give them credit instead of cash/check/etc
- You can do a Refund, and credit them with Store Credit
- You can add Store Credit from the Customer page (New > Store Credit), which will not be associated to a transaction
Then in the Customer Purchase, you can "pay them" in Store Credit - and it will maintain a balance that is available to use on future invoices.
Invoice Refunds will work in a similar way, you can pay it out to Store Credit and it will be available to use in the future.
Then you'll see the credit balance on the Customer Detail page like this
Now on an Invoice, you can click "Apply Credit" and it will look like this
Old "Customer Credit Balance"
Go to the Customer Detail screen and click edit:
Then, on the Customer Edit screen, click the Additional Fields tab and adjust the balance: