Hello, @anthonysceam,
1. I have just tested your site on my personal mobile phone and it is working perfectly fine, here is the video for you: https://www.veed.io/view/adade14c-5a90-4135-8e0c-9883c99e6f2d?sharingWidget=true
It must be the cache issue at your end nothing else, please try to clear the browser cache and cookies and check back, otherwise please try to check the site on any other mobile phone, I am sure you will not face this issue anymore.
2. Please copy and paste the below-given CSS code to XStore >> Theme Settings >> Theme Custom CSS >> Global CSS and check back to your site after removing the browser cache.
.popup-added-to-cart .upsell-products .content-product .product-title {
font-size: calc(var(--product-title-size-proportion) * 0.8rem) !important;
}
Screenshot: https://postimg.cc/wRQwcdcg
3. Unfortunately no, you can not Modify/Add the content to that popup as that is the default one, so it requires extra Customization.
Regards 8Themes Team.