Exchanges already extract per order commissions. You do not pay per message (so add, cancels and amends are free, you only pay when you get traded [1]). A per-

Exchanges already extract per order commissions. You do not pay per message (so add, cancels and amends are free, you only pay when you get traded [1]). A per-message would probably significantly affect existing strategies and greatly increase spreads, but I don't think it would prevent all forms of ULL trading. [1] But even there exchanges offer rebates, if not outright incentives, for market makers to provide liquidity.