I was going to use the padded envelope for most of my things and it is 5.30. Things like this that are so light will probably be cheaper to not use the flat rate boxes, but I'll double check when I get to the post office.
I always used the flat rate 5.30 envelope, until I found out that most of the time it's WAY cheaper to weigh the item and use a padded manila envelope. They cost $1 for two at the dollar store, so what I do is charge an extra .50 for the envelope when posting the flat rate fee. Better than $5.30 right? :P