Tire pressure monitoring systems

I have the TST510. Two of my sensors have gone bad. Have to mail them back and then they will send me new ones. Kind of a pain. Why can't they send and I send my bad ones at the same time? Oh well eventually I will switch to tiremender. I have only had for 16 months. Says batteries are good for 5 to 7 years. Yeah right. Just hope the other four don't go out the same time. But, then again if they do might be time to purchase another brand.