StripeInvoice addLineItem not working

[fedegiust]

Hi,

I'm quite new to salesforce and got a task using your Stripe Client.
I've been trying to do the following.

Once an invoice is created on stripe, to use the webhook invoice.created and then add a line item to the invoice that was just created before process the invoice on Salesforce.

I'm trying now to use the addLineItem method but nothing gets added, and get nothing on the logs.

I've tried calling it like this

            StripeInvoiceItemResponse li = invoice.addLineItem(3);
            System.debug('### Invoice Line Item invoice: ' + li.invoice_id);
            System.debug('### Invoice line item id: ' + li.id);   

or simply

    invoice.addLineItem(3);

Nothing gets added to the invoice on stripe so I don't know exactly what am I missing.

Thanks in advance