Ah, I am seeing that happen now when I make the width as small as possible. It’s jumping to the left at the last moment.
That div is simply using a “0 auto 0 auto” margin to make it centered within whatever div it’s in. There must be something about your theme that’s causing that to break based on the specific view width.
It seems like it has something to do with the Proceed to Checkout button because when I get very small on the width I see the PayPal buttons go left aligned with the Proceed button, but the Proceed button is wider than the PayPal buttons at that point. If I go as far as possible then the Proceed button becomes the same size as the PayPal buttons and they looked centered again.
So again, there must be something about the theme layout causing that div to go left aligned when it reaches a specific width. I tried to find a quick rule that might fix it with Inspector, but I’m not seeing. I can’t spend too much time on something like that without requesting a premium support order, but you might be able to get a quick tip from the theme support team.