Center absolute positioned div

If i understood your question then it will work as below.

You can do it in three ways.

No1 - Using position. Apply width 100% to button parent div and apply the button style as bellow.

.buy-btn{
    display: inline-block;       /* Display inline block */
    text-align: center;          /* For button text center */
    position: absolute;          /* Position absolute */
    left: 50%;                   /* Move 50% from left */
    bottom: 10px;                /* Move 10px from bottom */
    transform: translateX(-50%); /* Move button Center position  */
}

No2 - Using parent div, apply width 100% to your parent div and remove the postion absolute from button.

.parentDiv {
    width: 100%;            /* for full width */
    text-align: center;     /* for child element center */
 }

.buy-btn{
    display: inline-block;       /* Display inline block */
    text-align: center;          /* For button text center */
}

No3 - Using margin, apply width 100% to your parent div and remove the postion absolute from button.

.parentDiv {
    width: 100%;            /* for full width */
 }

.buy-btn{
    display: inline-block;       /* Display inline block */
    text-align: center;          /* For button text center */        
    margin: 0 auto;              /* For button center */
}

/* Replace below css  and add position relative to its parent class*/

.buy-btn {
       text-align: center;
       position: absolute;
       bottom: 10px;
       left: 50%;
       display: inline-block;
       transform: translateX(-50%);
    }