1.
Always ensure the ink cartridge is compatible with your specific franking machine model. Each franking machine has it's own unique franking ink cartridge and in most cases it only fits your specific model.
2.
Always ensure the franking ink used is Royal Mail authorised. Due to the high cost of franking ink, many companies try to pass off and sell generic ink. Whilst it is cheaper franking machine ink, it is cheap for a reason and will cause damage.