Paypal is not listed as an accepted method of payment in this ad. If you look at the ad and note the Payment Forms Accepted description, it states See Item Description. In order for a Paypal link to be generated when you send the invoice, you would have had to select Paypal as a payment method when you originally submitted the ad. This is why when you send the invoice on this ad, the option to select Paypal on the invoice is grayed out.