Can I substitute fusible interfacing for sew-in interfacing?
Now that I've seen the pattern, I would say that for the handles to stand up by themselves, as shown in the pattern, 




and for the bag not to flop over, as they are wont to do, hair canvas would be much more effective than normal interfacing (whether sewn in or fusible).  I've seen hair canvas used in tailored jackets (not the modern "deconstructed" kind) to give body to the shoulders, collar and lapels.

Here's how Pellon describes it:


  Sew-in Hair Canvas Interfacing is an excellent tailoring product. It is a woven interfacing that is suitable for garment construction where crisp detail or firm control is desired. Use with medium to heavyweight fabric.


When I was googling hair canvas I came across a fusible kind which, if Pellon is selling it, should work similarly to the sew-in kind.

If the shop where you live doesn't sell hair canvas, you could ask them what they can recommend as a substitute, showing them the sketch and photograph of the bag.  Basically, you need something appropriate for creating a crisp, tailored look.



I recommend making a full lining even if the pattern doesn't call for that.  It's much nicer to rummage for some small object that's floating in the bottom of the bag if there's a nice, full, smooth lining.  

I take back what I said about fusing to the lining.  For this kind of bag the lining should be soft and flexible.

By the way, I believe fusible interfacing can in principle be sewn in.