Since #13666 our fee calculation assumes that ECSDA signatures are a maximum of 71 bytes, rather than 72. However, not all hardware wallets implement R-value grinding.

This can cause us to underpay fees when using an external signer. That's especially problematic when the user picks 1 sat/vbyte as the fee, because it won't even get into our own mempool.

The easiest solution would be to modify `ExternalSignerScriptPubKeyMan` to assume 72 bytes for ECSDA signatures.

Slightly more advanced would be to expand HWI to keep track of which devices and firmware versions (if any) support R-value grinding. We'd then have to store that in the wallet in some new field. Personally I'd rather work on better Taproot support, which thanks to Schnorr avoids this issue altogether. 
/**
 * Select all coins from coin_control, and if coin_control 'm_allow_other_inputs=true', call 'AutomaticCoinSelection' to
 * select a set of coins such that nTargetValue - pre_set_inputs.total_amount is met.
 */
util::Result<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& available_coins, const PreSelectedInputs& pre_set_inputs,
                                          const CAmount& nTargetValue, const CCoinControl& coin_control,
                                          const CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
struct CreatedTransactionResult
{
    CTransactionRef tx;
    CAmount fee;
    FeeCalculation fee_calc;
    int change_pos;
    CreatedTransactionResult(CTransactionRef _tx, CAmount _fee, int _change_pos, const FeeCalculation& _fee_calc)
        : tx(_tx), fee(_fee), fee_calc(_fee_calc), change_pos(_change_pos) {}
};
/**
 * Create a new transaction paying the recipients with a set of coins
 * selected by SelectCoins(); Also create the change output, when needed
 * @note passing change_pos as -1 will result in setting a random position
 */
util::Result<CreatedTransactionResult> CreateTransaction(CWallet& wallet, const std::vector<CRecipient>& vecSend, int change_pos, const CCoinControl& coin_control, bool sign = true);
/**
 * Insert additional inputs into the transaction by
 * calling CreateTransaction();
 */
bool FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& nFeeRet, int& nChangePosInOut, bilingual_str& error, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, CCoinControl);
} 
#endif