Tolong bantu saya saya mendapatkan error

"Fatal error: Uncaught Error: Object of class stdClass could not be converted to string in ... Stack trace: #0 {main} thrown"

<?php

if (isset($_POST["checkout"])) {

            echo $d->id;

            $id_ongkir = $_POST["id_ongkir"];

            $tanggal_pembelian = date("Y-m-d");

            $ambil = $conn->query("SELECT * FROM ongkir WHERE id_ongkir='$id_ongkir'");

            $arrayongkir = $ambil->fetch_assoc();

            $tarif = isset($arrayongkir['tarif']);

            $harga = $conn->query("SELECT * FROM db_product WHERE product_price='product_price'");

            $totalpembelian =   $p->product_price + $tarif;

$conn->query("INSERT INTO pembelian (

                id,id_ongkir,tanggal_pembelian,total_pembelian)

                VALUES ('$d','$id_ongkir','$tanggal_pembelian','$totalpembelian')");

}

        ?>

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