It is very unlikely that a wrong sticker is purchased since there is only one sticker available valid for all vehicle types of 3.5 tons or smaller.
In some cases, only if never been affixed to a vehicle, a reseller might take the unused sticker back out of goodwill.
An affixed sticker is never refundable.
Related articles:
Can I remove and reuse a physical Swiss vignette (Sticker)?
Can I get a refund for a destroyed Swiss sticker?
How long is the Swiss sticker valid?