One way to reduce fees is to encourage usage of Replace-By-Fee, BIP 125 [0]. It allows wallets to recommend lower fees, because if a transaction gets stuck due to underestimation, the fee can easily be bumped.

Bitcoin Core has had support for RBF for a while, and as of v0.15.0 recommends lower fees [1] when the user chooses to use RBF.

I recently submitted a pull request that would turn on RBF by default, which triggered some discussion [2]. To ease the transition for merchants who are reluctant to see their customers use RBF, Matt Corallo suggested that wallets honor a no125=1 flag.

So a BIP-21 URI would look like this: bitcoin:175t...45W?amount=20.3&no125=1

When this flag is set, wallets should not use RBF, regardless of their default, unless the user explicitly overrides the merchant's preference.

Afaik adding this flag won't break existing BIP-21 support. It doesn't use the req- prefix, because it's optional. I'm also not aware of any ad hoc standards that use no125 in BIP-21-ish URIs.

- Sjors

P.S. I'd similarly suggest adding a bech32 param, but that's for another discussion

[0] https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki
[1] https://bitcoincore.org/en/2017/09/01/release-0.15.0/#better-fee-estimates
[2] https://github.com/bitcoin/bitcoin/pull/11605
[3] https://github.com/bitcoin/bitcoin/issues/11828