Попробуйте приведенный ниже фрагмент. В этом я изменил
.shop:hover>span,
.shop:focus>span,
.shop:active>span {
color: white;
}
На
.shop:hover>a>span,
.shop:focus>a>span,
.shop:active>a>span {
color: white;
}
.button {
background-color: white;
border: 2px solid transparent;
border-radius: 2em;
color: #eff6f9;
display: inline-block;
font-family: Merriweather, Arial, sans-serif;
font-weight: 300;
font-style: italic;
font-size: 0.7rem;
font-weight: bold;
letter-spacing: 1px;
line-height: 1;
padding: 1em 2em;
text-shadow: none;
transition: background-color 0.125s ease-in;
margin-top: 3em;
}
.wrapper{
height:200px;
width:200px;
padding:10px;
background-color:yellow;
}
.button a {
font-size: 2em;
text-decoration-line: none;
color: black;
text-decoration: none
}
.button .fa-w-18 {
color: black;
margin-left: 5px;
}
.button:hover,
.button:focus,
.button:active {
background-color: transparent;
border-color: white;
color: white;
}
.shop:hover>.fa-w-18,
.shop:focus>.fa-w-18,
.shop:active>.fa-w-18 {
color: white;
}
.button:hover>a>span,
.button:focus>a>span,
.button:active>a>span {
color: white;
}
a:focus,
.button a:hover {
text-decoration: none;
}
<div class="wrapper">
<div class="button" id="bu">
<a class="shop" href="https://app.nmpl.store/">
<span>Order Now</span>
<i class="fas fa-shopping-cart"></i>
</a>
</div>
</div>