topnotch_ordersystem/app/Filament/Admin/Resources/InvoiceResource/Pages/CreateInvoice.php

<?php

namespace App\Filament\Admin\Resources\InvoiceResource\Pages;

use App\Filament\Admin\Resources\InvoiceResource;
use App\Models\Invoice;
use App\Models\TaxRate;
use Filament\Resources\Pages\CreateRecord;
use Illuminate\Database\Eloquent\Model;

class CreateInvoice extends CreateRecord
{
    protected static string $resource = InvoiceResource::class;

    protected function handleRecordCreation(array $data): Model
    {
        $data['pst_rate'] = TaxRate::where('name', 'PST')->value('value');
        $data['gst_rate'] = TaxRate::where('name', 'GST')->value('value');

        $invoice = Invoice::create($data);
        $invoice->calculateTotals();

        return $invoice;
    }
}