Unfair debt

I was owing Pharmacy direct R900.00 and they kept on sending me bill on sms and I requested an email so that I can have proof of the debt and be able to include the paid amount on my tax return, they delayed sending the mail but finally did. By the time I paid them they had already handed me to the lawyers who I never had contact with before paying. Bredel contacted me I told them I had paid but they insisted I must send proof that I paid I requested that they get proof from Parmacy Direct the lady told me she will show me, now the cost has doubled even after I paid they charged me interest. I see this asan ******** debt because their representative threatened me and continued to add interest an call charges that I never even received. I need the word to go out there because this is unfair.