single grid price component || CSS Grid Flexbox

Hi guys! I have few questions:

1. Is it good to use in desktop version css grid then in mobile version change it to flexbox using media queries? It's my first time with grid and I have no idea how to use it properly. Frist what came to my mind is to change it in flexbox but I don't know if it is the right choice.
2. How to do this trick with price $29 to make it big then "per month" lighter?
3. Why can't I change the fonts to be lighter? In section "why us" the fonts in the design looks different then on my computer.