http://cryptova2lamba4zzsfeqldil6jrst46bnoxz5l4jgitfstecpamlryd.onion/faq.html
Why should I set custom fee? This is done in order to ensure your privacy and safety. In the event that someone knows your service fee, they will be able to analyse the publicly available ledger called Blockchain to see how and when your transactions were made and figure out the destination account.