It sounds like you purchased fish without knowing they would outgrow your tank. In those circumstances I would go back to the seller first and tell them what you now know and request nicely that they take the fish back. Ideally they would refund you/give you store credit but most importantly you need them out of your tank so don't prioritise this.
If you bought them knowing they might not be compatible with your tank, ideally you would discuss this at point of sale - I personally have returned a few fish purchased on the basis that if they don't fit in with my ickle tank they will be accepted back presuming they are in good health.
Basically, I wouldn't just turn up with the fish in a bag initially - try and agree a return. If they duped you on eventual size however and still won't accept a return I would find a few ways to make their life awkward including asking to speak with management and making my point politely but frequently whilst in their premises.
