.btn-cantidad{
    position:fixed;  
    /* mover boton*/
    bottom:70px;
    right:20px;  
     color:#ffffff; 
    font-size:25px;   
    z-index:100;
    transition: all 300ms ease;
}

@media only screen and (min-width:320px) and (max-width:768px){
    .btn-cantidad{
        width:63px;
        height:63px;
        line-height: 66px;
    }
}