<a class="powered-by-footer" 
  href="https://gumroad.com/" target="_blank">
  <span class="logo"></span>
  Powered by Gumroad
</a>

.powered-by-footer {
border-radius: 4px;
background: #fff;
padding: 10px 15px 10px 12px;
font-size: 14px;
bottom: 15px;
left: 15px;
position: fixed;
box-shadow: 0 4px 4px 0 rgb(0 0 0 / 10%);
text-decoration: none;
font-family: -apple-system,\.SFNSDisplay-Regular,Helvetica Neue,Helvetica,Arial,sans-serif;
}

.powered-by-footer .logo{
background: url(https://assets.gumroad.com/packs/media/logo/logo-74072d46.svg) 0 0 no-repeat;
display: inline-block;
height: 16px;
width: 17px;
margin-right: 8px;
vertical-align: text-bottom;
}