How can I ensure the security of my payment system via PayPal?

How can I ensure the security of my payment system via PayPal?

I use the vue-paypal-check create the frontend PayPal button for the payment.

the code is bellow:

  <Pay-Pal
    v-if="paypal_live_id && paypal_sandbox_id"
    :amount="amount"
    currency="USD"
    :client="credentials"
    :env="paypal_env"

    @payment-authorized="payment_authorized_cb"
    @payment-completed="payment_completed_cb"
    @payment-cancelled="payment_cancelled_cb"

    :items="pay_items"
  >

</Pay-Pal>

some dota is bellow:

data(){
  return {
    paypal_env: this.$GLOBAL_CONST.PAYMENT.PAYPAL_ENV,

    paypal_sandbox_id: undefined,
    paypal_live_id: undefined,
  }
},
computed: {

  credentials() {
    return {
      sandbox: this.paypal_sandbox_id,
      production: this.paypal_live_id,
    }
  },
},

the callback method of pay success:

  payment_completed_cb(res){
    some method to access API for payment success // there will request the API for change the order status or reduce the balance. 
  },

but I have a question, if someone of customer is evil with technology, he call the payment_completed_cb directly, not pass the paypal payment.

How can I prevent this?

from How can I ensure the security of my payment system via PayPal?