Help button is connected with SnapEngage chat. Really nice feature.
We take only 5%, traffic is not limited, if your file is getting attention we copy it to our server not to drain your Dropbox bandwidth. Dropbox and PayPal account is enough. We are working on other file hosting services (Box.net & Google Drive), too. Some people don't love Dropbox as mush as we do :)
Payment for file goes directly between customer and yours PayPal, so PayPal is sending email notification that you got $x from user and description which file was bought is added as a note.
There is no limit on file size. Anything what can be uploaded to your Dropbox.