Subjunctive Mood

Subjunctive Mood
 

In the sentence below, I conjugated the verb “pesar” in the subjunctive, but I don’t know why; it only “feels” right.

Is it correct?

Is the indicative permissible in this sentence?

Ofrecemos entrega al día siguiente para documentos urgentes y paquetes que pesen hasta 50 kg.

Any grammatical explanations are appreciated.

The subjunctive mood is correctly used in this case, since you're talking about an unknown (uncertainty).
Feeling uncertain about something is one of the reasons to use the subjunctive. Desires, emotions, uncertainty/possibility/subjectivity, and anything contrary to fact trigger the subjunctive.

Thanks, Rusty.

I'm glad my "feeling" was correct.

Thanks for your explanation.

I'm glad the explanation was useful.

Another way to think about it is that the packages you're referring to are not specific packages, they are "packages in general".