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.
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2016-03-10T22:11:46.390
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question
grammar translation
Does 筈はなくて grammar construction have a negative connotation?
The key here is the idea that the backpack is unusual, nearly magical, in how
much it can contain despite its apparent small size. The finishing phrase:

> そんな都合のいいものの筈はなくて

筈{はず} in this context refers to the speaker's _expectation_. The なくて here is a
conjugation of ない _"not"_ , so yes, this has a negative connotation. Much as
in English advertisements, this full phrase could be translated as _"there's
**no way** this could be so good!"_

Another more prosaic example of 筈 usage (usually written in kana) in a
positive context might be あの手紙はもう届いた **はず** だ。 → _"The **expectation** is that
that letter has already been delivered."_ (literally) → _"I'm **pretty sure**
that letter has been delivered by now."_ (more idiomatically)
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2016-03-10T23:23:04.293
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answer