Did you buy a prepaid or postpaid SIM? Was the SIM activated? 

You could transfer your number to Koodo following thishttps://www.koodomobile.com/en/help/moving-your-number-koodo

It’s a post paid sim (month to month). I assume it was activated as the retailer said it could not be returned. 

I’m not able to make a new self serve account for some reason. When I click the register button, it brings me to the help page. I was previously a Koodo customer and my old account doesn’t work.

You cannot activate a postpaid SIM yourself. You can only do that with prepaid. Your old postpaid account was cancelled and you can't reactivate there. 

If you want a postpaid service, you will have to get a plan (online or instore). After that you will have access to a new selfserve account. 

When I try to buy a plan online, it is automatically adding a $5 sim card, but I already have a SIM I bought today. Would I be able to just use the one I have here?

I don’t care about receiving a second SIM, but I’m leaving the country in a day and would need to activate before, so I can’t wait for one to arrive in the mail which is why I went out and bought one today. 

No, There isn't any option to remove the $5sim online. It is the way koodo setup. Once you receive the activation email from Koodo, you can however visit a Koodo kiosk and ask someone there to put the existing SIM on the new account. By doing that you might not have to wait for shipping from Koodo. 

@PhoneHelp098 Where you bought the SIM the person should have opened a new line for you.  Since it’s postpaid account a credit check was to be run and plan selected along with an agreement printed. It’s strange if they didn’t do it as they are not commissioned on plain SIM sales.  Did they think you were looking for a replacement SIM? 

As far as activating the one you have now pls schedule a callback at koodo.com/chat and a rep can assist you over the phone. 

You can order online. Koodo will mail you a sim once you are approved and send you a self serve confirmation email. 

You can then login to self serve and swap your sim in self serve to your new sim. The sim swap will need to send you a confirmation. Choose the email option.

This may take a day but you can do this even if you are out of country, just bring your koodo sim.with you 

Can you have a trusted friend or relative send the SIM card over to the address you are going to stay in another country once it arrives at the address listed on your Self-Serve account?