I found the construction 筈はなくて and by helping me with the context, I suppose to have properly translated but I am not sure.
This backpack has been done in a special way, it is a magical bag that you can put many more items than what appears to be possible to put in it ... but it does not turn to my advantage!
If it is filled beyond its limits, things come out and break.
Of course, the important thing is that I am alive and well.
Yeah, is not an object of your dreams, it is not easy to handle!
Is it correct my interpretation of そんな都合のいいものの筈はなくて?
I think that the subject is denying the fact that the backpack is convenient.