Bought some of that premaid stuff in a plastic bag/jar (I think Mae Ploy or Aroy-D). Used about half and was curious how long it would last in the fridge. Doesn't seem to have any preservatives, but probably has a lot of salt.
Turned out fabulous by the way. Followed some of the tips on another thread about using the coconut cream on top to simmer with the paste, and added some fish sauce, lime and palm sugar at the end. Great flavor.