your domain provider is ? 

In any of them.

Hi You can use any third app . and if you want that by custom coding , I can do it for You .

As i seen on website , that white space taken by card__reviews  block add this css in headercard__reviews {display: block;line-height: 1;margin: 0 0 0 0;min-height: 0 !important;position: relative;}

nope , not working. 

Unable to see your site .

hi If you still not get solution , please check in 'product-grid-item.liquid' page for  View details and change replace it with buy now

To achieve this , login to shopify admin->customize->product page In sidebar theme setting and variant selection options.