It probably makes sense to inspect the styles to see where the difference is, or if any style is being overwritten. Does the problem also occur in Gutenberg content? Have you already contacted surecart support?
Your findings prompted me to check whether the issue persists in a private Safari browser window. It does not — the text is vertically centered as expected. I then logged out of WordPress, and the issue was resolved there as well.
It seems the issue only occurs when I’m logged into WordPress. Interestingly, I hadn’t noticed this behavior before. The same effect is also present in both Brave and LibreWolf when logged into WordPress.
Okay, then it should be easy to spot the culprit by right-clicking » inspect on the button. Then, you can see which styles apply and where they are coming from.
As mentioned in my previous email, the issue was caused by the admin bar. The text within the button was offset vertically by the exact height of the admin bar. When viewing the website without the admin bar, the text aligns correctly within the button.