Hi Jennifer,
It looks like J2Store component on your site is loading Bootstrap 2 version, however, the template (JA Artsy) you're using uses Bootstrap 4 so there are lots of conflicts in style.
You can add this css to solve this issue:
.j2store-product-list .j2store-products-row.row-fluid .span4 {
width: 33.33333%;
float: left;
margin-left: 2%;
}
But I can't guarantee issues on other pages as these 2 versions are totally different. It's best if you can contact J2Store if their component can support Bootstrap 4 or not. As you know Joomla 4 is coming and it uses Bootstrap 4 also.
Regards