Grand Total Function Returns the Wrong Grand Total

Question

This function returns the wrong grand total. When I add items to the cart, it returns a much higher grand total than is actually there. How can I fix it?

You can see the app in action at cart-64d3f.firebaseapp.com

In the component:

totals = [];

get grandTotal() {

let i;
let sub_total = 0;
let grand_total = 0;
if (this.isSubmitted == true) {
if (typeof this.product_price !== "undefined" && typeof this.quantity !== "undefined") {
sub_total = this.product_price * this.quantity;
this.totals.push(sub_total);
}
}
for (i = 0; i < this.totals.length; i++) {
grand_total += this.totals[i];
}
return grand_total;
}

In the HTML:

<td>{{grandTotal | currency:'USD':true }}</td>