Almost always, the reason for the over payment is because the transaction was done in the wrong currency.
You got charged 100$ USD when it should have been 100$ CDN. This would show up on your statement as approx. 128$CDN + banks will add a service charge to foreign exchanges making it seem like you got overcharged.
You got charged 100£ for an order that was $100 USD. This would show on your statement as 137.77$USD + banks will add a service.
How do transactions get charged in the wrong currency?
This can happen because you are in the wrong store view. For example, you are in the USA store instead of the Canada store. An easy way to check is to look at the flag beside the logo and make sure you are in the right store.
This also happens when you use a saved credit card that was saved in another currency. If you place an order in the wrong store, and choose the save credit card option, then that card will forever process transactions in that currency, even if you choose a different currency or store, the saved credit card currency overrides all other currency settings.
To fix this
- Delete the saved credit card which was saved with the unwanted currency
- Fill out a refund request form for the difference