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<?php

add_action( 'woocommerce_check_cart_items', 'lpb_free_shipping_cart_notice' );
function lpb_free_shipping_cart_notice () {
  if ( ! is_cart() ) return; 

  // mude o valor 150 para o valor mínimo do frete grátis
  $free_shipping_amount = 150;
  
  // modelo da mensagem (não remova o "%s")
  $message = 'Compre mais %s para ter frete grátis!';
  
  $cart_amount = WC()->cart->cart_contents_total;
  $diff = $free_shipping_amount - $cart_amount;
  
  if ( $diff > 0 ) {
	wc_print_notice( sprintf( __( $message, 'lpb-free-shipping-cart-notice' ), wc_price( $diff ) ), 'notice' );
  }
}