Not an attribute offer as an addition

[Ipood]

Hello,

I would like to offer battery addition (Not included in the price) in a drop down box. Currently, I set it up as an attribute but I ended up with doubled stock number, since I had to create two options.

For example, I am selling a toy and it can be purchased with 3 x AAA batteries.
I have 100 toys that can be sold with or without batteries. When the toy is sold out "with batteries" I still have 100 more toys "without batteries".

I do not want to sell batteries on the side. I would like to offer it while the customer is selecting the products color. and then adds battery, as an option not something that will change the characteristic of the product.

I wonder if you guys can shed some light on my little problem here.

Thanks.