gopay-inline-change-credentials.php

<?php

add_filter( 'gopay_inline_client_id', 'toret_change_client_id', 10, 2 );
function toret_change_client_id( $client_id, $order_id ){

    $order = wc_get_order( $order_id );
    $shipping = $order->get_items( 'shipping' );
    $shipping = array_shift( $shipping );
  
    if( $shipping->get_method_id() == 'flat_rate' ){
      return 'my_custom_client_id';
    }
  
    return $client_id;

}

add_filter( 'gopay_inline_client_secret', 'toret_change_client_secret', 10, 2 );
function toret_change_client_secret( $client_secret, $order_id ){

    $order = wc_get_order( $order_id );
    $shipping = $order->get_items( 'shipping' );
    $shipping = array_shift( $shipping );
  
    if( $shipping->get_method_id() == 'flat_rate' ){
      return 'my_custom_client_secret';
    }
  
    return $client_secret;

}

add_filter( 'gopay_inline_goid', 'toret_change_goid', 10, 2 );
function toret_change_goid( $goid, $order_id ){

    $order = wc_get_order( $order_id );
    $shipping = $order->get_items( 'shipping' );
    $shipping = array_shift( $shipping );
  
    if( $shipping->get_method_id() == 'flat_rate' ){
      return 'my_custom_goid';
    }
  
    return $goid;

}