Returned Equipment - still sent to Collections

In March I had cancelled my internet service with Telus as I was switching providers. As soon as my new internet was installed, I returned the telus equipment right away via Canada Post.

As this was sent back so long ago, I have since discarded the receipt Canada Post gave me. I assumed everything was fine, as Telus never contacted me regarding a balance and when I logged into my account, it said there were no accounts assigned... no mention of a balance.

Last week, I happened to sign into my CreditKarma and noticed I had been sent to collections for a $115 balance as of this month. At no point was there ever anything in my mail, email, or voicemails left regarding said balance from Telus. I haven't even heard from said collections company?!

I called Telus right away, they refused to help me as I didn't have the tracking slip anymore. I have since disputed with TransUnion and filed a complaint with CCTS. I asked Canada Post if there was any way to recall the proof of me sending it - they said no.

Has anyone else had this happen? Any suggestions on other avenues I might try to have this corrected? It's ​damaged my credit significantly, so this is super disappointing.

Is it possible this can be corrected?