Apple IIc ROM4x, IIcPlus ROM5x
8.00USD

In ROM4x, Enable Smart Port and add self-diagnosis function and more.
Ideal for upgrading from ROM255.
To replace the ROM, in addition to replacing it, you will also need to change the solder jumpers in two places (W1 and W2). This is to enable the A14 address.*Requires a soldering iron and a utility knife. *Detailed concept of jumper pads on the IIc can be found here.
 
IIcPlus ROM5x does not require resetting the jumpers on the pads, just replace the ROM chip. It also has a downclocking function, which eliminates the drawbacks unique to rare machines and makes them easier to use.

*Detailed concept of jumper pads on the IIc can be found here.