When the total value of a basket is £0 you are not able to Checkout as normal. Instead select basket options by tapping on the icon at the top right depicting three lines.
Now select 'Complete Basket'. A warning message will appear saying that the basket is not completely paid for because you have not been through the normal checkout process. Select 'Complete' and the next empty basket will be displayed ready for you to continue using Shopwave.
You may also wish to use this function to register products that have not been paid for but have been sold. It will show in your reporting that payment was not taken.