Shortcode do not work

First of all, I must admit I ‘m a it puzzled why you would need an extra button when it’s already there by default
i.e the prizes themselves are essentially buttons and – by default(unless turned off) – under each price there’s a cart icon and the size name you could style whichever way you like to make them appear more like buttons.

However, that’s more of an observation than a judgement. You can of course do as you please.

Back to the issue at hand:

By default wppizza post content does not execute shortcodes. If you need that , you have to enable it in
WPPizza->Layout->General: Shortodes in menu items content

(It would be my guess that this is the problem, but let me know if this does not solve the issue of course)