How to get rid of this open space on my product page? (Mobile)

14 minutes ago, sophiamd57 said:

I'd like to get rid the the space between my product description and the "extra info" on my product page for mobile.

You can do this with some CSS.

By default, there are 40 pixels of margin at the bottom of the Product Excerpt section and then another 5vw units of margin on top of the Additional Info section. To reduce the whitespace you can reduce one or both of these to meet your requirements. I've set them both to zero in this example CSS:

@media only screen and (max-width: 799px) {
    .ProductItem-details .ProductItem-details-excerpt {
        margin-bottom: 0; //40px
    }
    .ProductItem .ProductItem-additional {
        margin-top: 0; //5vw
    }
}

Notes:
1. This is for Squarespace 7.1.
2. This uses a media query so that it only applies to mobile devices (devices with a screen width of 799px or smaller).
3. CSS should be added to Design > Custom CSS.