The bad thing about that jr. heater is that if even once you let the water get below that water mark the heater will shut off...FOR GOOD. There's no turning back on. You may want to think about a larger tank, as they're so much easier to deal with...you can get a real heater to fit in there too
I'm sorry for your losses...
Have you read up on cycling and do you have ammonia,nitrite and nitrate test kits? If you've sanitized the whole tank then the tank probably builds up ammonia and other nasties pretty quick unless you're doing daily water changes.
You might have also gotten a bad batch of bettas at that store...maybe try looking for one at another store that seems somewhat active and has bright colors.