Mobile shopping cart malfunction

SITE
chicagochamberchoir.org

ISSUE
Is specifically related to our ticketing page: chicagochamberchoir.org/single-tickets. On mobile, after adding the first item(s) to the cart, the user cannot add anything else. This issue occurs only on mobile; everything works perfectly on desktop.

CUSTOM CSS
I have custom code to make the shopping cart icon operate the way I'd like. I think that's what's causing the issue. Can anyone see where I've gone wrong?:

---

header{position:fixed!important;z-index:9999;width:100%;-webkit-box-shadow:0px 3px 15px rgba(100,100,100,.49);-moz-box-shadow:0px 3px 15px rgba(100,100,100,.49);box-shadow:0px 3px 15px rgba(100,100,100,.49)}
 /* Cart */
 .Cart {
   transition: 0.35s;
 }
 .sqs-pill-shopping-cart {
   display: inline !important;
   opacity: 0.95 !important;
   position: fixed;
   top: 110px;
   right: 20px;
 }

---

The cart pill that normally takes up very little space in the lower corner, now fills the page, covering the entirety of the screen, preventing anyone from reaching any of the controls or buttons. It's because of a combination of the CSS you've added and the CSS that is built into the Commerce styles.