Learn Portuguese (Portugal): O fecho vai rebentar porque a mala está demasiado pesada! - The zipper is going to burst because the suitcase is too heavy!

In this sentence, demasiado acts as an adverb modifying the adjective pesada (meaning "too heavy"). Adverbs never change their endings. You only make demasiado agree with gender and number when it acts as a quantifier describing a noun (meaning "too much" or "too many"), such as in demasiada roupa (too much clothing).

Why does the sentence use está instead of é to say the suitcase is heavy?

We use está (from the verb estar) to describe a temporary state. The suitcase is heavy right now because it is overpacked. If you used é (from the verb ser), it would imply that the suitcase itself is made of a heavy material and is heavy even when completely empty.

Is vai rebentar the only way to say "is going to burst"?

You could also use the future tense rebentará (will burst). However, using the present tense of ir (to go) plus an infinitive verb — in this case, vai rebentar — is the most natural and common way to express an immediate or certain future event in conversational Portuguese.

I know fecho means "lock" or "closure". How do we know it means "zipper" here?