Rücksendeetikett, Rücksendeformular und Zahlungsbeleg liegen im Paket.

In German, liegen is often used for things that are physically lying somewhere.

So Rücksendeetikett, Rücksendeformular und Zahlungsbeleg liegen im Paket literally means something like:

The return label, return form, and proof of payment are lying in the package.

In natural English, we would usually just say are in the package, but German often prefers a more specific verb of position:

• liegen = lie, be lying
• stehen = stand, be standing
• sitzen = sit, be seated
• hängen = hang

For paper items like a label, form, or receipt, liegen sounds very natural.

Because the subject is more than one thing:

• Rücksendeetikett
• Rücksendeformular
• Zahlungsbeleg

These three nouns together form a plural subject, so the verb must also be plural:

• singular: Das Rücksendeetikett liegt im Paket.
• plural: Das Rücksendeetikett und das Rücksendeformular liegen im Paket.

So liegen is the correct plural form of liegen in the present tense.

German often leaves out articles in contexts like lists, labels, instructions, packing information, or business language.

So instead of saying:

• Das Rücksendeetikett, das Rücksendeformular und der Zahlungsbeleg liegen im Paket

it is very natural and more concise to say:

• Rücksendeetikett, Rücksendeformular und Zahlungsbeleg liegen im Paket.

This style is especially common in product information, order confirmations, and shipping or return instructions.

The meaning is still understood as referring to specific items.

im is a contraction of in dem.

• in = in
• dem = the (dative singular, usually masculine or neuter)

So:

• im Paket = in dem Paket = in the package

German very often uses these contractions:

• im = in dem
• am = an dem
• zum = zu dem
• zur = zu der

In this sentence, im Paket is the normal and natural form.

Because this sentence describes a location, not a movement.

German distinguishes between:

• where? → usually dative
• where to? → usually accusative

Here, the items are already inside the package, so this is location:

• im Paket = in dem Paket → dative

Compare:

• Die Unterlagen liegen im Paket.
  The documents are in the package.
  (location)

• Ich lege die Unterlagen in das Paket.
  I put the documents into the package.
  (movement toward the inside)

So im Paket is correct because nothing is being moved in this sentence.

Yes. German very often creates long compound nouns by combining smaller words.

Here is how they break down:

• Rücksendeetikett
  Rücksend(e)

  • Etikett
    = return label / return shipping label

• Rücksendeformular
  Rücksend(e)

  • Formular
    = return form

• Zahlungsbeleg
  Zahlung

  • Beleg
    = proof of payment / payment receipt

A few things to notice:

• The last part usually tells you the basic category of the word.
  For example, a Rücksendeformular is a kind of Formular.
• German compounds are written as one word, not as separate words.
• Sometimes a linking element appears, such as the -s- in Zahlungsbeleg.

Learning to spot the parts of compound nouns is a very useful skill in German.

The subject is the whole list:

Rücksendeetikett, Rücksendeformular und Zahlungsbeleg

All three nouns together are the things that liegen.

You can think of it as:

• [Return label], [return form], and [proof of payment] = subject
• liegen = verb
• im Paket = location

So the basic sentence structure is:

Subject + verb + place

This is very normal German word order for a statement.

Because they are all nouns, and in German all nouns are capitalized.

So in this sentence:

• Rücksendeetikett = noun
• Rücksendeformular = noun
• Zahlungsbeleg = noun
• Paket = noun

This is one of the most noticeable differences from English. In English, only proper nouns are normally capitalized, but in German, all nouns are.

Yes, grammatically you could say:

Rücksendeetikett, Rücksendeformular und Zahlungsbeleg sind im Paket.

That means simply are in the package.

However, liegen im Paket sounds more natural when talking about physical paper items that are inside something. It gives a clearer picture of their position.

So:

• sind im Paket = correct, more neutral
• liegen im Paket = also correct, often more idiomatic here

Yes, Paket is a neuter noun:

• das Paket

You can tell this in the sentence because:

• im = in dem
• dem can be dative singular masculine or neuter

Since Paket is neuter, im Paket comes from in dem Paket.

Useful forms:

• nominative: das Paket
• accusative: das Paket
• dative: dem Paket

That is why im Paket is correct.

No, it would not be wrong. In dem Paket is grammatically correct.

But in normal German, im Paket is much more natural because contractions like im are very common in everyday speech and writing.

So:

• im Paket = natural, standard
• in dem Paket = correct, but more explicit or slightly heavier in style

Most native speakers would choose im Paket here.