I'm not sure what you get for $50/month, but I had an ebook site a few years ago, and I wrote a custom script that automatically watermarked the PDF with the Pa

I'm not sure what you get for $50/month, but I had an ebook site a few years ago, and I wrote a custom script that automatically watermarked the PDF with the PayPal email address of the buyer. It was just plain PHP so I could host it on my $6/month unlimited domain shared hosting along with 50 other sites I had there, making the cost negligible.