Am not sure that sitting on it will help all that much, after all.. it's already age-old... seriously though, if you're doing mixing of homebrew things, letting nutes / supplements sit around for months can cause issues as they pick up humidity/moisture/potential bugs/etc depending on what you're stockpiling. Only way I keep anything more than 2 weeks that's 'open' from a sealed bag is to put said bag in another bag, then flush the O2 out of it with nitrogen before resealing... just a bit anal about that, but would rather my shit be as I intend it to be when the bag is reopened. Your mileage may vary greatly.
Let me rephrase, if I did buy it I would just have it for awhile considering the size. Do you think it would be ok to leave it sitting around until it is all used?