I do know that if you fuse the interfacing, you need to be certain that the fabric washing/drying temperatures will be compatible with that of the interfacing. For example, I used a fusible interfacing in a cotton top for my daughter. It was darling until it went through the dryer. Then it was a mess that couldn't be ironed out. The cotton could be dried on high heat, but not the interfacing. The interfacing shrunk, but was adhered to the cotton. Therefore the shirt had permanent puckers and wrinkles. If you won't be putting the bag in the dryer, there wouldn't be an issue. I don't know if NOT fusing the interfacing would take care of the heat issue.

Do you have a pattern number and what type of fabric are you using for the bag?

I suspect that the interfacing is to keep the bag and strap from sagging. I did appliance covers, didn't interface, now have sagging covers because I didn't follow pattern directions to interface.

I would try a small test sample by fusing a 4x4" swatch according to manufacturer directions, do a test seam, dunk it in water, and dry the swatch with the blow dryer.

I've never had good luck with fusibles, though I know a lot of people swear by them. I worked for a drycleaner for a number of years and we ALWAYS had claims for sport coats that had "bubbling" of fused interfacing. I decided then and there to by-pass it and sew it in when I was making something.

And it's becoming increasingly difficult to find the sew-in variety; either the woven or the nonwoven! I usually buy mine through my tailoring/alteration supplier because the shops near me never seem to have it. And they NEVER have hair canvas for lapels/collars.

Interesting question.

Thanks for the advice everyone. I have had the bubbling effect before so I was afraid of it happening. I found the sew in type at Joann's yesterday so I used it. It looks fine and has plenty of body. I'll post a picture to the gallery if I can figure out how.

I posted a photo but it looked blurred on my computer.