public function avoir($zechallenge_id, $product_id, $quantity = 1)
{
return Model::FacturationAvoir()->create(['status' => 'UNBILLED', 'purchase_date' => time(), 'zechallenge_id' => (int) $zechallenge_id, 'product_id' => (int) $product_id, 'quantity' => (int) $quantity])->save()->id;
}
