The "background" properly is missing a quote at the end of the URL.
You have the following at the moment:
background: url('https://phxhandsan.com/wp-content/uploads/2020/05/PAYPAL.png);It should be the follwoing:
background: url('https://phxhandsan.com/wp-content/uploads/2020/05/PAYPAL.png');Try changing that to see if there is a difference.
BTW: Your checkout button looks fine to me. It is how it is normally supposed to be. I checked using my iphone mobile device and it looks fine to me. We used that exact same button on our site and we never had any customers complain about it. Maybe you can ask the customer to send you a screenshot so we can see if they are really seeing something we are not seeing due to the type of device they are using.