Customers can request specific fonts but this service is not always free.
If the customer chooses a Google Font from the following link: https://fonts.google.com/ then they would be able to customise their fonts. Typically the template websites are set to have two specific fonts (one for headings and one for body text). Further changes or ‘testing’ fonts would incur charges, be sure the customer is aware of this.
Also if the font is not on Google Fonts and the customer is supplying it directly, we need to have proof that the font has been purchased for Commercial Use to avoid any legal action by the creator.