Sometimes invoices in Felix show VAT (international sales tax abbreviation) instead of GST. To Change VAT to GST you need to follow this process:
In the settings column on the left select - BOOKING ENGINE > PROPERTY BOOKING PAGE > DEVELOPERS
Go to the 'Custom Text' setting and add the following:
VAT|GST
Vat|GST
You can then click on the icon shown below to copy the setting to all properties: