From bitcoin newest source, how can I set transaction fee as more lower?
Some wallet(ledger nano hardware wallet) even provide now custom transaction fee, how is possible?
fees are optional.
in the past "eg. last Dec and Jan" fee was so high and maybe you need to pay more than the transaction. but Mempool now is empty "you can send using 1 sat/b".
you can use this sites to know a suitable fee
https://jochen-hoenicke.de/queue and
https://bitcoinfees.earn.com/#delay
generally, segwit reduce fees and native segwit (Bech32) give you lower fee than P2SH (38% less)
so using wallets support Bech32 segwit will give you the lowest fee (Bitcoin Core, Electrum and ledger nano hardware wallet)