The PayPal plugin is not simple, but the tooltips and the wiki should tell you most of what you need to know.
The plugin redirects the user to the standard PayPal payment page (on PayPal's site). The user then logs in to PayPal independently, it doesn't use any email or credentials supplied through the Fabrik form. When they pay, the PayPal IPN mechanism fires off a call back to your site, providing you with details of the transaction, which the Fabrik plugin responds to, and updates the forms data accordingly with things lie the txn-id, status, etc.
As you are just using a simple fixed cost, you don't need to worry about any complexities with evaluating that. Just put 16 in the price box.
The important part is the IPN fields. It's fairly essential you provide (hidden) fields for the ones marked "Optional but STRONGLY advised", and definitely at least txn-id, status, and payment. Those will be filled out by the plugin after payment is processed. Without those, you have no record of the transaction.
I also suggest getting yourself a PayPal sandbox, so you can test without hitting your real merchant account.
https://developer.paypal.com/developer/accounts/
... which then lets you set up test merchant and customer accounts, and the Fabrik plugin lets you select the sandbox to test with rather than the live site.
As you have multiple payment methods, you will also need to provide a "Condition" which only returns true if PayPal is the selected method. Which in your case would be something like ...
Code:
return '{advertise_your_trade_or_ser___payment_method_paid_via_raw}' == 'xx';
... where xx is the value you assign to the new PayPal option.
-- hugh