I always pack snacks & cereal. I use tupperware containers and everything arrives fine.
Just DO NOT pack peanut butter. Your luggage WILL be opened and searched. The density is the same as explosives and the xray machine will pick that up.
Also, flying from Canada, you cannot bring some fruits and veggies over the line. (You can look on the US Customs web site, but I know tomatoes are a definate no no, that and Kinder Suprise chocolate eggs

)
Have a wonderful time!
I was curious about the Kinder Suprise and went to Wiki and found this:
Prohibition on sale or import into the United States:
While Kinder eggs have been available in Canada since 1975, the product has never been allowed into the US for several reasons. In 1993 the Ferrero Group (the maker of Kinder eggs) applied to have the eggs sold in the USA, but was turned down because of a prohibition against having an inedible item inside an edible object. More recently, the US Consumer Products Safety Commission determined in 2008 that the product did not meet the small-parts requirement for toys for children under the age of three, creating a choking and aspiration hazard in young children. Since 1991, at least 7 children worldwide have died of choking after swallowing the toy inside the Kinder egg[6].
On January 10, 2011, it was reported that a Kinder egg was seized from a woman's car during a random inspection while entering the United States from Canada. She was informed that bringing the egg into the United States could result in a $300 fine.[3] Seizures such as that one, and even the prohibition of the product itself, have been cited on political blogs and web forums as an example of an overzealous application of law and an intrusion upon personal liberty.[7] The rationale against a ban of the product also takes the form that deaths have been too few for it to be considered a serious danger. Additionally, the argument is made that there should be a consistent standard in place, with several worse dangers not being regulated, making it anomalous that a Kinder egg should be regulated
To answer OP's original topic:
I would say if you have to pay for shipping such as a luggage fee,etc, it's probably best to just do Garden Grocer or buy in the parks or shops. That would depend on how long you are staying and how many actual snacks you require. Over the years I've found that we don't eat that many snacks and often end up carting them back home and in worse condition then they arrived, smushed etc. A few things are good for us such as a box of cereal, then we buy a gallon of milk and our own coffee product as disney coffee is like dishwater to me.
If you are planning entire meals then carry on - the savings are probably worth it.