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.
このバックパックは特殊な製法でできていて、見た目の大きさよりずっと多くのアイテムが収納できる魔法の袋…とまぁ、そんな都合のいいものの筈はなくて、
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!生地の限界がくれば溢れるし破れる。
If it is filled beyond its limits, things come out and break.勿論重さも健在だ。
Of course, the important thing is that I am alive and well.夢のような便利なアイテムはないんだよね。
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.