A client recently asked for a better way to simplify his PayPal check out process but also collect additional information from the user during check that was related to the service his is providing. I found this useful page from the PayPal Getting Started Section. The additional text can be used for a variety of reasons.
The PayPal Getting Started example shows how to add a text field to a Donation button but the same process works for the Buy Now button as well.
Here is a sample PayPal Button I created in my merchant account.