Should we charge customer for extra days spent to development/customization? I read two source and get confused.

first, this statement " It's not as easy as it may seem as customers often think they need custom developments. On the other hand, implementation service companies are happy to charge extra service/development days for these customizations. But I have to warn both sides, custom developments are not good for you!"

second: check sign in "odoo online success packs" for development and customization

thank you in advance