Fizz keeper pumps

The most economical way to buy soda is to get it in those big 2 liter bottles. The problem with this is that if you don’t drink all the soda within a couple of days, it looses its fizz, and then you end up having to pour flat soda down the sink. So much for saving money. But there is an invention out there that fits onto the bottle, and re-pressurizes your soda. Some are pumps and some are squeeze bulbs. Either way, they can apparently keep your soda fizzy for weeks.
Fizz keeper pump:

Fizz keeper pumps pictured:
Left: Jokari Fizz-Keeper Pump & Pour, 3 units
Right: Jokari Fizz-Keeper (2 liter), 3 units