I'm not very clear on the objective that you have laid out. It seems that you are wanting to pass along a custom field to the checkout that will be retained as part of the info for the order. Is this correct? You mention that you would like the user to download this in a history report. Please post back and explain a little more about what you are hoping to accomplish. I think with a little more detail we will be able to help guide you along. Also include info about your checkout weather it is Payments Pro or another PayPal checkout.