all-battery products are cheap because most of them are just low cost, entry level units with less development, testing, and product support than other "name brand" products. The chargers may not be as reliable as you may want. That said, a battery is a battery. If you do order use one of their codes like "tenergydeals" or such (just google it) to save $.
You can solder whatever connectors you want or even just use alligator clips (not recommended though). If you do replace the connectors be careful to cut ONE battery lead at a time so as to not short the battery.
If you're just looking for AA type batteries you can just get them anywhere locally. That is, if your robot isn't using powerful motors requiring high output batteries, you don't need to invest in a good set of batteries and charger.