Hi Robyn,
I think you will find the echo vacs actually have a white (well when new) inner bag that fits inside the black outer bag.
The reason for this is primarily as a secondary dust filter, it is also able to be washed and also gives the zip some added protection from dust etc.
I have had one for nearly five years, have never had a problem with the zip or outer bag but have replaced one inner bag (easily available from your echo dealer).
Cheers Ricardo123