Bag patterns by American Designers refer to interfacing available in the US and this can be confusing for anyone living in the UK, we do have the same or very similar interfacing just with different names.
Below is a list of the main interfacings used in these patterns which are available at Bobbin Girl Bag Making Supplies together with the names of US versions and manufacturers.
UK Version US Version
Vilene H250/305 Pellon 808 Craft-Fuse
Vilene S520 Pellon Peltex 71F
Vilene G700 Pellon Shape-Flex 101
If you're still bewildered by the world of interfacing, please don't hesitate to contact me, I will do my best to help you