It's not so bad, it's 3/4 of a gallon which is quite big for most store selling standards, they'll usually coax you into buying something like 1 liter big or those little decorative teeny tiny cups. I've heard of bettas living comfortable in 1/2 gallons too. As for the flowers, well it's just like a plant and at least you can feed the betta and it has air and the water can be changed, unlike the "betta in a vase sustainable ecosystem" bit.
I wouldn't buy it, I'd rather make it at home. I had a similar setup, in a 1.5 gallon bowl I had a plant sticking straight out of it, before I decided to trim it back.