You need to speak to customer service at the Walmart location where you purchased the top-up voucher. 

I did but they couldn’t help and referred me to Koodo website. They said they had these kind of issues before. 

I am still not sure what happened.

So you bought voucher from Walmart. How much did you buy and what did they tell you? When you mentioned “a few”, did you buy them at once or at several occasions?
 

Did they actually charge you for them all?

Yes, so to be exact I bought 9x 10$ vouchers but the cashier only scanned the first one and gave me the pin for that and charged me for all of them by multiplying by 9!

So the code was for $90 or it was a code for $10? If it was just the code for $10, you had to go back and ask to speak to a manager there. They should be able to check.