<style>
.responsive-iframe {
width: 100%;
height: 100%;
max-width: 100%;
max-height: 100%;
border: 0;
}
.iframe-container {
position: relative;
width: 100%;
padding-bottom: 150%;
height: 0;
}
.iframe-container iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
</style>
<div class="iframe-container">
<iframe class="responsive-iframe" src="ショップURL" allowfullscreen></iframe>
</div>