I would not use those bags because they are not food grade. I believe that mylar itself is non toxic but they are usually coated with plastic. Some plastics are and some are not toxic. Plus some packing materials are treated with various compounds for things like insect protection etc. These would be toxic. Also you have no idea what they hjave picked up from what was shipped in them.

But if you are interested in mylar type bags they make them to hold milk and those infamous wine in a box deals. You may be able to find a local source that would give you empties for free. Just need cleaning. You might be able to find a source to get food grade bags.