Sometimes when making changes to the website it's necessary to place "test" orders to ensure everything is working OK.
While voucher codes can be created in Shopify, they can only be created for either "Free Shipping" or discount on the products. As a result a payment by credit card or PayPal is still required, e.g. for $10 for shipping.
As a work around you can activate the COD (cash on delivery) payment method. Note this will be displayed for all customers at checkout so only activate it during the period of time you will be doing testing.
To activate COD you need admin access and do the following:
Login
Go to Settings
Go to Payment Providers
Scroll to Manual Payments and activate Cash On Delivery COD.
In the "Enter Additional Details" add the text ****** NOT CURRENTLY AVAILABLE ****** (this is added so that any real customers would be steered away from selecting this as a payment option while you are testing the website)
Click Save / Activate
Do the testing you need to do.
As soon as testing is complete login back in and Deactivate the COD by clicking "Edit" > "Deactivate"