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;
 }

---