After a year to two years, the baking powder in the mix will spontaneously activate. The whole premise of baking powder making baked goods rise, is when it activates it produces carbon dioxide... i.e. the little bubbles in your pancake batter. That, along with the steam in the mixture, makes the batter rise as it cooks. That will make your mylar bag "swell" up. It will also mean when you do use the mix, it will not rise (think very flat crepe-like pancakes.) You can add more baking powder when you use the mixture (1 1/2 tsp. per cup of dry mix), but you'll also need to add about a 1/2 tsp. of sugar per cup to that mixture or the double application of baking powder will make it slightly bitter tasting.
Baking soda does, however, lose its efficacy. My understanding is that it loses some of its potency as it ages, but is still safe to eat. I read somewhere that if you mix it with vinegar and it still bubbles pretty good, it will work. I’ve tested some that I’ve had for over 4 years and it’s till good.
Like Back40 said, the best way is to store baking soda and cream of tartar. Both have other uses in the kitchen also, so storing them separately gives you versatility. Then when you use them, use 1 tsp. baking soda, 1/2 tsp. cream of tartar per cup of plain flour.
I have used powdered shortening. I like the product but there are some things you have to consider. I didn't include the powdered shortening option in the recipe I posted because of an ongoing mother hen problem I have. Back40 has small children. All humans need fat in their diets but small children, most especially do, because they are growing and they do not have the reserves that adults do.
Shortening powder works in baking and in cooking. It doesn't add very much fat to the diet and can't be counted as a fat source. Depending on where you get it from it can also have some things in it you may or may not want to have in your diet.
Honeyville.com has a little bit better taste to me, but it has corn syrup solids added to it and the increased amount of fat on the nutrition label is from trans fats. Trans fats are part of regular shortening, they are a result of what happens to the vegetable oil when the hydrogenation of turning oil from a liquid into a solid occurs. Transfats do not go rancid nearly as fast as regular fat.
Beprepared.com, which markets under Emergency Essentials brands, also has a powdered shortening, minus the corn syrup and the transfats. It doesn't have quite the taste, but it doesn't have quite the negative ingredients either. The Emergency Essentials brand also lasts longer in storage, because those fats are removed.
To use powdered shortening, for each 1/3 cup shortening you need, use 1/2 cup shortening powder and 1/3 cup water. That will make just a hair over 1/3 cup but without using scales to cook, that reconstitution will work.