Backseat Baby Alarm
What does it cost?

$29.99/unit+ $4:00 shipping for 1 up to 4 units anywhere in continental US.  If you live in Canada cost for 1 unit with shipping included is $38.70, for 2 is $70.94,  3 are $102.32, and 4 are $136.11. We ship international as well.. 
Can be it used in any type of vehicle,  like mini-vans or pick-ups? Can I put my child in and out from either rear door?

Yes and yes. 
How long will the batteries last?

At least 18 months and most probably longer depending on the climate and usage. It does not have a low battery indicator but the volume will go down if the batteries are getting weak, just like a flashlight bulb gets dimmer. Always put in new, brand name alkaline batteries! It takes 3 AA's.
Can the volume be adjusted?

No, by design.
Is it reliable?

No device is foolproof, including this one. We can only explain as we mentioned above that the basic technology which senses the opening of the doors are "reed sensors" and these are the exact same components used in millions of home security systems to detect an intruder opening a window or door. Because of this solid track record, we chose to use the same thing for the Backseat Baby Alarm. Secondly, we purposely hardwired the sensors to the alarm, avoiding using any kind of wireless transmitter. Why? Because 3 other companies before us, including a multi-million dollar car seat company with plenty of resources, used wireless radio transmitters with poor results. When outside agencies (Children's Hosp. of Philadelphia and Consumer Reports) tested their products, they actually recommended that parents NOT use them as they gave a false sense of security due to their unreliability. Among other things, the problems experienced included the transmitter and receivers decoupling from each other and the device giving off false alarms while the tester was driving around. By wiring the components directly to each other, the Backseat Baby Alarm is immune to outside interference. 
Won't I get used to the sound?

We think it's extremely unlikely. Undoubtedly you have heard the ringtone of your cell phone thousands of times. But have you ever once been sitting there and you cell phone went off but your brain did not register it going off because your brain got so used to the sound?   

Probably not. In fact, probably the opposite will happen. Why? It's because our brains are trained by repetition. Probably every time you hear the Backseat Baby Alarm's unique music box melody,  you will be with your baby and that constant reinforcement of "music box sound" = "baby in car!" will probably work powerfully to train your brain to automatically think "BABY!" anytime you hear it.
How does it work?

The simplest way to explain it is it works just like when your car makes a beeping sound when you leave your keys in the ignition - only the Backseat Baby Alarm is reminding you of something far, far more important.

It turns itself on by your opening either of the back doors to put your child in and then after that, plays a music box chime when you open the driver's door. No loud sirens, honking horns, etc. Just a simple "audio tap on the shoulder" as a gentle reminder to "look before you lock". After that when you open the back door to take your child out, you make it go back to sleep till the next time  by simply tapping the Off/Sleep switch located on the top edge of either rear door. 
Hardwired for reliability and price!
A simple reminder chime so you don't forget to "look before you lock".
How does this compare with other similar products?

There are not many products available but these are what we have found and prices on their websites as of 05/18/15:

Suddenly Safe N' Secure Pad - $79.95*
Baby Alert International "Soft Clip" - $49.95*
Baby Alert Internation "Child Minder"  Pad - $56.95*
Small One Safety - $54.95 (Was available but check of website makes it seem it is not available at this time and might still be undergoing beta and market testing)
Sunshine Baby - $99.00~$129.00
    *reviewed by NHTSA/Children's Hospital of Philadelphia. Click to see article on report's assessment here\
Why is the Backseat Baby Alarm so much less?

It's simple. The components we were able to use to make our system work are much, much  cheaper than the components the other products had to use. In order to work, all alarms like this have to include at least 2 sensors - one to sense the child and one to sense the driver - AND some means for them to communicate to the actual alarm to "tell" it to go off. Our sensors are "reed sensors", which are the exact same thing used in home security systems to detect the opening of a window or door so they are produced in the millions each year and therefore, very cheap. Secondly, the way our sensors "talk" with the alarm is with good old copper wire. The other systems use a radio transmitter and a radio receiver. So yes, the other products might be more "hi-tech" and offer different capabilities,  but we think ours is just as effective and actually, probably uses the most reliable technology. 
Is there a warranty?

Yes, you may try it out and return your unit for any reason within 30 days of purchase and receive a refund of your purchase price. If you don't like it, return it. In addition, there is a limited 1 year limited warranty against defects in manufacturing. Click here for the legalese.