Try, for the love of god to convince your manager NOT to give goldfish for those tanks. Not even ONE.
By keeping the betta in a 1 gallon tank you are basically replicating a puddle of death. Presuming your usual types of customers are the sorts I'm thinking of, they are looking for something easy to look after in terms of tank and fish yes? Well a goldfish in a 5 gal and a betta in a 1 gal is the complete and utter opposite of that aim. The smaller tha tank, the harder it is to keep stable and safe for the fish due to ammonia build up and lack of swimming space . So the store's current offer is completely useless for the needs of the customer.
Explain that the 5 gallon tanks are perfect for the
BETTAS ( one male only for the tank ) , and offer some sort of possible discount on the fish when buying a tank.
Explain that bettas absolutely do NOT live their lives permanently in puddles and hoofprints in the wild ( often a reason people give for keeping them incorrectly in tiny containers ) . Many thousands die in them as they dry up in the dry season. The lucky ones who manage to end up in slightly deeper ruts and pools use their labyrinth organ
as a survival method only.
Being able to breathe atmospheric air as a means of trying desperately to stay alive is not a reason to keep a betta in a tiny stagnant tub,
and it does not make them magically immune to the ammonia produced in their waste the same as all fish.
They need space, and they need a
gentle filter.
A properly cared for heated and filtered five gallon will give a longer living, much happier fish, and will not take up more than 14 inches of space if the tank is square or rectangular so it's hardly a vast tank taking up loads of room.
As for the one gallon tanks, why not convince the store to sell them as easy shrimp or sea monkey setups? They are fine for that, and cherry shrimp do not require a heater. They produce very little waste and can get by with water changes twice a week if the 1 gal has no filter.