Request refund or use the R400 to cover my outstanding bill as I refuse to pay a late fee as no one is willing to assist me

I was unable to redeem my Shopritemoneymarket voucher, I reach out to payjustnow on many platforms they fail to assist me, I even said the R400 they can take to cover my outstanding they just never get back to me, unable to call and also chat system is not very helpful, can a manager please call me as I am a dissatisfied customer and refuse to pay any late payments fees