A mixing cartridge delivery system, which mixes additive with base liquid flowing through it, utilizes a dispensing closure valve to achieve improved flow of mixed liquid from the cartridge. A closure base may include a tethered lid. A flexible valve, such as a slit valve, is supported on a valve supporting end of the closure base and retained thereon with a valve retainer that interlocks with the closure base. The system may include a one-way valve for base liquid and a mixing section defined in the cartridge. The dispensing closure valve achieves improved flow characteristics in combination with the base liquid valve. The dispensing closure valve also prevents leakage and inadvertent discharge from the cartridge system.