def estimate_fee(tx_size: nil, network: @network, fee_per_kb: nil)
      unspents = inputs.map(&:output)
      Fee.estimate(unspents, outputs,
                   network: network, tx_size: tx_size, fee_per_kb: fee_per_kb)
    end
def estimate_fee(tx_size: nil, network: @network, fee_per_kb: nil)
      unspents = inputs.map(&:output)
      Fee.estimate(unspents, outputs,
                   network: network, tx_size: tx_size, fee_per_kb: fee_per_kb)
    end
Estimate the fee in satoshis for this transaction.  Takes an optional
 tx_size argument because it is impossible to determine programmatically
 the size of the scripts used to create P2SH outputs.
 Rough testing of the size of a 2of3 multisig p2sh input: 297 +/- 40 bytes