Mango Airlines non-compliance with their own refund voucher

Due to a Mango Airlines flight, a Mango Airline representative personally collected me from the boarding gate on the 25/08/2019 and transferred me to an SAA flight as I would have missed my international connecting flight in Johannesburg. Due to Mango Airlines not offering paying for additional luggage, I had to pay for additional luggage per kilo which amounted to R1375. Mango Airlines then gave me a refund voucher as they do not refund you at the airport. (King Shaka International- Durban). I explained to Mango Airlines that I needed to pay SAA again for the additional luggage and was assured that the refund voucher would enable me- well, a refund. After numerous e-mails and phone calls back and forth, Mango Airlines is still asking me for a reason as to why I am requesting the refund. They are now saying the voucher has an expiration date. They are now not responding to any of my emails and have failed to comply with their own policies. I have document proof of flight details, emails and dates to corroborate any of the above.