Getting the Product from an Invoice

[Larry Amiel]

Hello,
I am new to WHMCS and I am currently building a Payment Gateway for it. The Payment Gateway requires to determine whether the product I have in the invoice is a Recurring or a One-Time payment type. I am currently accessing the Invoice using the $invoiceId provided...
How am I able to access the Product model to be able to grab the $paymentType variable to determine the payment type?

If I understand correctly this is how I can get the Invoice Object. So I just need to know how I am able to grab the Payment Type.

$invoice = Invoice::find( $invoiceId );

Thanks.