Gavin Andresen - 2015-04-03 20:52:54

@s{quotedtext} @s{quotedtext}
You can't, the reference implementation wallet always pays some-amount-per-1000-bytes-of-the-transaction.

The rules for the 0.10 release are:

+ By default, you always pay a fee (this is different from previous releases that would send transactions without a fee if they had high enough priority)  (run with -sendfreetransactions=1 to get the old behavior of sending high-priority transactions without a fee)
+ By default, the fee-per-kilobyte is estimated based on recent transaction confirmation history.

To get close to what you want (pay a fixed fee per transaction), run with -paytxfee=0.whatever  : that tells the wallet code to pay 0.whatever BTC per 1000 bytes. Most transactions are about 500 bytes big.

See here:  http://core2.bitcoincore.org/smartfee/fee_graph.html  ... for how high to make -paytxfee=0.whatever based on how long you're willing to wait for the first confirmation (that page graphs estimates from the latest&greatest fee estimation code from Alex Morcos that will hopefully be in the 0.11 Bitcoin Core release).