Utilize a mask to achieve transparency while also incorporating an inner shadow in place of the outline
.customer-box-shadow {
box-shadow: 0 0 0 2px inset red; /* instead of outline */
padding:50px;
-webkit-mask:
/* adjust the 60% to control the height of the cut */
linear-gradient(#000 0 0) right/2px 60% no-repeat,
linear-gradient(#000 0 0);
-webkit-mask-composite: xor;
mask-composite: exclude;
}
body {
background:linear-gradient(90deg,#ccc,#fff)
}
<div class="customer-box-shadow">
CONTENT
</div>
If outline is necessary, explore using clip-path
for a similar effect
.customer-box-shadow {
--d: 2px; /* outline border */
--c: 60%; /* the cut */
outline: var(--d) solid red;
padding:50px;
clip-path:
polygon(calc(-1*var(--d)) calc(-1*var(--d)),
calc(100% + var(--d)) calc(-1*var(--d)),
calc(100% + var(--d)) calc(50% - var(--c)/2),
100% calc(50% - var(--c)/2),
100% calc(50% + var(--c)/2),
calc(100% + var(--d)) calc(50% + var(--c)/2),
calc(100% + var(--d)) calc(100% + var(--d)),
calc(-1*var(--d)) calc(100% + var(--d)))
}
body {
background:linear-gradient(90deg,#ccc,#fff)
}
<div class="customer-box-shadow">
CONTENT
</div>