Sewing a book bag and wallet

So I've been practising the sewing again, so that I can line some of my knitted bags. I finally finished an old project, a simple book bag with a vintage striped piece of fabric I bought at a flea market. I lined it with some hot pink cotton. I also made a little matching wallet since I had some little scraps left over. It was a pretty small piece of fabric. This bag is perfect for rolling up and taking along in case of surprise shopping.

I'm planning to sew the lining of the turquoise bag next, but I'm not sure about using the interfacing. I'm not sure if I should just use a strip at the top, or for the whole bag. I don't want the whole thing to become too stiff and the interfacing is kind of thick. The woman at the sewing store said it would be good for a bag. Plus, does the interfacing cover the entire fabric area including seams? Because then the seams would be really thick. If there are any sewers reading this, I'd really appreciate some help!