Paid up letter

I’ve been trying to get a paid up letter for over 2 months after having extra interest added just after I had settled my account. The worst part, I need a paid up letter to clear my account and I asked the payjustnow agents of this was a legitimate way of paying my account and they said it was. Now I’ve paid it up and there’s no reflection of that anywhere. My nimble profile looks it has never had any accounts and there’s nothing under the “settled accounts” tab.