purchase-overlay.php

<?php

function sc_bottom_overlay() {
  	$button_text = 'Purchase';              // Button Text
	$link_uri = 'http://YOUR_LINK_HERE';    // Url for button Link
	$overlay = '<div class="overlay"><span style="opacity:1;"><a class="overlay-button" href="'. $link_uri .'">'. $button_text .'</a></span></div>';
	echo $overlay;
}
add_action( 'genesis_after_footer', 'sc_bottom_overlay' );