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. 
_("This is the minimum transaction fee you pay on every transaction."));
        }
        walInstance->m_min_fee = CFeeRate{min_tx_fee.value()};
    }
    if (args.IsArgSet("-maxapsfee")) {
        const stdstring max_aps_fee{args.GetArg("-maxapsfee", "")};
        if (max_aps_fee == "-1") {
            walInstance->m_max_aps_fee = -1;
        } else if (stdoptional<CAmount> max_fee = ParseMoney(max_aps_fee)) {
            if (max_fee.value() > HIGH_APS_FEE) {
                warnings.push_back(AmountHighWarn("-maxapsfee") + Untranslated(" ") +
                                  _("This is the maximum transaction fee you pay (in addition to the normal fee) to prioritize partial spend avoidance over regular coin selection."));
            }
            walInstance->m_max_aps_fee = max_fee.value();
        } else {
            error = AmountErrMsg("maxapsfee", max_aps_fee);
            return nullptr;
        }
    }
    if (args.IsArgSet("-fallbackfee")) {
        stdoptional<CAmount> fallback_fee = ParseMoney(args.GetArg("-fallbackfee", ""));
        if (!fallback_fee) {
            error = strprintf(_("Invalid amount for -fallbackfee=<amount> '%s'"), args.GetArg("-fallbackfee", ""));
            return nullptr;
        } else if (fallback_fee.value() > HIGH_TX_FEE_PER_KB) {
            warnings.push_back(AmountHighWarn("-fallbackfee") + Untranslated(" ") +
                               _("This is the transaction fee you may pay when fee estimates are not available."));
        }
        walInstance->m_fallback_fee = CFeeRate{fallback_fee.value()};
    }