Sorry that I didn't reply earlier, I must have missed the notification.
All your styles are written directly into the code instead of keeping them in separate CSS files. I would recommend elaborating on this; I'm sure it will help to keep your code clean and more flexible in the future.
Also please note that this is not a problem caused by s2Member. If you want to display the three boxes pretty on phones, please move all the inline style into CSS files as I suggested.
In the picture below I highlighted what style you should add for the buttons to keep them in the boxes.