You can only store breast milk on each pump i.e. one milkbag per pump. You can not add the newly pumped milk (which is warm) into a bottle already containing a refrigerated breastmilk (which is cool). It won't last long.
However, you can combine the breastmilk when it is stored at the same temperature.
I stored in a steralised glass jars because no BPA. Make sure your bottle is BPA free.