Being billed for something that i returned 3 weeks ago

After being sent a pocket wifi that I didn't order by Buymondo, I dealt with a lovely lady that helped me return the item, cancel the application(I didn't every apply) and I was promised that any money owing would be returned and no deductions would come off my account. The case was closed and I was happy. Until this morning when R185 was deducted from my account for said item that I didnt order, returned and case was closed! Vodacom and Buymondo I am not impressed! How do I get my money back? How do we stop future amounts being deducted from my account?