You know that Samourai provides three preset fee profiles, but sometimes you want ultimate control over your transactions. You can choose to provide a custom miner fee instead of relying on one of the preset fee profiles. This is pretty advanced so only do this if you know what you're doing.
Tap to set your preferred custom fee
On the send screen tap the text that says "Your fee selection is set to:.."
You will be prompted to enter a custom numberical value in sat/b
The lowest custom fee value you may choose is '4'
Samourai will prevent you from drastically overspending on the miner fee as a fail-safe.