How to add a fixed transaction fee to each transaction?
If you want total control over the fee then you will need to build the transaction yourself using createrawtransaction - more info here: https://en.bitcoin.it/wiki/Raw_Transactions

However, it's important to note there was a relatively unnoticed change in the way paytxfee handles the fee calculation earlier in 2016 - see here: https://github.com/bitcoin/bitcoin/issues/7633