Your checkout page looks correct while you are logged in, but visitors see a broken payment form.
SiteGround Speed Optimizer skips optimization for logged-in users, so while you are logged in to wp-admin you see the page in its original state. Visitors receive the optimized version, which may have scripts combined or deferred in ways that break third-party payment forms.
Always test checkout pages in a private browser window while logged out to see what visitors actually experience.