Ruby-Rose

You should be able to modify the message by replacing the CSS with something like this:
#confirmed-page {   // Order Confirmed title   h1 {     font-size: 34px!important;   }   // Order details text   p {     font-size: 20px!important;   }   // Your purchase includes digital...   h3 {     font-size: 0!important;     visibility: hidden;   }   h3:after {     content: "To access your purchase, please click the link below. Thank you!";     font-size: 28px!important;     visibility: visible;     font-weight: bold;   }   // Link to digital content   a {     font-size: 20px!important;     color: blue!important;     border-bottom: 1px solid blue!important;   } }  
Update:
From 07 February 2023, this accepted solution will only work for gift cards, memberships and subscriptions. This is because Squarespace have replaced the Order Confirmation page with the Order Status page for physical, digital and service product purchases.
 

Add to Design > Custom CSS
.scroll { white-space: nowrap !important; }  

@SquareRefresh It worked perfectly!!!! :') Thank you thank you!!!!! 

@SquareRefresh It worked perfectly!!!! :') Thank you thank you!!!!! 

Add to Home > Design > Custom CSS
@media screen and (max-width:767px) { div#page-section-5efcc1eb502fee6d856f49a2>.row>.col:not(:first-child) { width: 50% !important; float: left !important; } }