Paypal Buttons

I have set up pay now buttons for all of my products. I have put the paypal button code into the products page for each button. I'm assuming that in order to use these I need to manually create an invoice (since I'm bypassing the shopping cart altogether). Just want to make sure that I'm doing this correctly.

PHP:

$invoice = Am_Di::getInstance()->invoiceRecord;
$invoice->setUser(Am_Di::getInstance()->userTable->load($user_id));
$invoice->add(Am_Di::getInstance()->productTable->load($product_id));
$invoice->paysys_id = "paypal";
$invoice->calculate();
$invoice->insert();


Is that all I need to do?

Thanks