Yes. InmateAid's services cover every prison, jail, and detention center in the United States, including small county facilities like Dunklin County. No facility is off the map.
That said, the specifics of what each service can do vary from facility to facility. Letter delivery works everywhere through the US Postal Service. Phone service savings depend on who the facility's contracted phone carrier is. Money deposit options depend on what the facility accepts. The core services are available regardless, but the exact benefit you get from any one of them is shaped by how that specific facility operates.
If you have a question about a particular service for Dunklin County, email aid@inmateaid.com and the team can give you a straight answer for your situation.