Apply a coupon depending on cart weight 

add_action( 'woocommerce_before_cart', 'snipjar_apply_coupon_cart_weight' );

function snipjar_apply_coupon_cart_weight() {
	$coupon_code = 'snipdeal';
	// if cart content weight is more than 15kg
	if( 15 < WC()->cart->get_cart_contents_weight() ) {
		if( ! WC()->cart->has_discount( $coupon_code ) ) {
			WC()->cart->apply_coupon( $coupon_code );
		}
	} else {
		WC()->cart->remove_coupon( $coupon_code );
		WC()->cart->calculate_totals();
	}
}