El paquete está sobre la báscula.

Because Spanish uses estar for location in almost all normal cases.

Here, El paquete está sobre la báscula is talking about where the package is, so está is the correct choice.

A useful rule:

• ser = identity, origin, characteristics
• estar = location, condition, temporary states

So:

• El paquete es pesado. = The package is heavy.
• El paquete está sobre la báscula. = The package is on the scale.

In this sentence, sobre means on, on top of, or over.

So sobre la báscula means the package is physically positioned on the scale.

Be careful: sobre can also mean about/concerning in other contexts:

• un libro sobre historia = a book about history

But in this sentence, the physical-location meaning is the one being used.

Spanish uses articles much more often than English does.

So even when English might say:

• Package is on scale in a note or label-style context,

normal Spanish usually prefers:

• El paquete está sobre la báscula.

The articles tell you the nouns are specific in the situation:

• el paquete = the package
• la báscula = the scale

Noun gender in Spanish is not determined only by the last letter, though endings often give clues.

• paquete is a masculine noun, so it takes el
• báscula is a feminine noun, so it takes la

Common patterns:

• nouns ending in -o are often masculine
• nouns ending in -a are often feminine
• nouns ending in -e can be either masculine or feminine

So you have to learn the gender with the noun:

• el paquete
• la báscula

Báscula commonly means a weighing scale in many parts of Latin America.

Depending on the country, you may also hear:

• balanza
• peso in some informal local usage
• other regional terms

But báscula is a very standard and useful word, especially for a scale used to weigh something like luggage, boxes, or packages.

So this sentence sounds natural in Latin American Spanish.

Yes. Encima de la báscula is also natural and often very close in meaning.

Comparison:

• sobre la báscula = on the scale / over the scale
• encima de la báscula = on top of the scale

In many everyday situations, both work.
However, encima de often emphasizes more clearly that one thing is physically resting on top of another.

So:

• El paquete está sobre la báscula.
• El paquete está encima de la báscula.

Both are good, though sobre is a bit more compact.

The accent marks show where the stress goes.

• está is stressed on the last syllable: es-TÁ
• báscula is stressed on the first syllable: BÁS-cu-la

The written accents are important because they show the correct pronunciation and sometimes distinguish words from other forms.

For example:

• esta = this
• está = is

So the accent in está is not optional.

A simple pronunciation guide is:

el pa-KE-te es-TA SO-bre la BAS-ku-la

A few notes for an English speaker:

• paquete: the qu sounds like k
• está: stress the last syllable
• sobre: the r is a light Spanish r, not a strong English r
• báscula: stress the first syllable

A rough English-friendly approximation: el pah-KEH-teh es-TAH SOH-breh lah BAHS-koo-lah

Yes, but the original order is the most neutral and common:

• El paquete está sobre la báscula.

You could change the order for emphasis, for example:

• Sobre la báscula está el paquete.

That sounds more marked or literary, as if you are emphasizing the location.

For everyday speech, the original sentence is the best default.

Yes, usually it is understood as a current physical situation: the package is on the scale right now.

That fits well with estar, which is used for location and states.

Even if the package stays there for a while, Spanish still uses estar because the sentence is about where it is located, not what it permanently is.