The cart is supposed to float on the top right corner of the page once a product is added.
The cart does not appear and so they cannot checkout.
I do know that one of them is using Chrome on a mac. All the customers are in the US.
Site URL: https://www.milestonedecalart.com/
Some of my customers are reporting that the cart isn't visible even with some product added to it.
I cannot reproduce on my end. I used the marquee theme.