So Called "outstanding" Account

I’ve been with Mellins I style for quite some time for both my glasses and biofinity contact lenses. I usually get my contacts then pay them over 2 months. I pay a box a month. On the date of Saturday, August 26, 2017 11:21 AM I got a phone call that my account is outstanding. My last box of biofinity I bought was Sept-Nov 2016. This means it took them a year to only call me about "outstanding" account. According to them my last payment date was 3 September 2016. On Saturday, August 26, 2017 11:21 AM I emailed them my bank statements to prove that I payed my boxes both 3 September 2016 and 2 November 2016. I never got any feedback about my account regarding that. On 6 February 2018 I received a phone call, message and email about this "outstanding" account from Barrett & Associates. I insisted they send me a statement showing my last payment before I send my bank statements showing my proof that I payed 2 November 2016. Once again they told me my last payment was 3 September 2016. I went in to Bayside Mall Mellins I style to ask them what is going on. The lady told me that my account has been scratched off their system and handed over to Barrett & Associates and I should contact them in a day to follow up. I send them 3 emails on 6 February 2018 to ask them to get back to me ASAP 1 email including my Bank statement to show my payment on 2 November 2016. Never got a reply back. Send them another email on 7 February 2018 to ask them to please send me a feedback regarding the "emergency phone call" I had to make to them. Not so important if they can’t get back to me is it. On 8 February 2018 I still have no feedback regarding this account. From 2016 to 2018 calling about an outstanding account when I never even got a single feedback to what is going on.