Avsenderen ba meg la være å kaste pakkelappen før de hadde fått den i retur på postkontoret.

Avsenderen means the sender.

The base noun is en avsender = a sender.
When Norwegian adds -en to many masculine/common-gender nouns, it makes them definite:

• en avsender = a sender
• avsenderen = the sender

So the ending -en is functioning like English the.

Ba is the past tense of å be, which means to ask, to request, or sometimes to pray depending on context.

Here it means asked:

• å be = to ask
• ba = asked

So:

• Avsenderen ba meg ... = The sender asked me ...

This is an irregular verb, so learners often need to memorize it rather than build it from a regular pattern.

After å be when you ask someone to do something, Norwegian often uses object + infinitive without å.

So:

• Hun ba meg komme. = She asked me to come.
• Avsenderen ba meg la være å kaste ... = The sender asked me not to throw away ...

This is similar to English patterns like asked me come in structure, although English requires to there. Norwegian often does not.

You may also see be noen om å ..., but in this sentence the pattern is:

• ba meg la være å ...

That is completely normal.

La være å means to refrain from, to avoid, or more naturally in many contexts, not to do something.

So:

• la være å kaste = not throw away / refrain from throwing away

It is a very common expression in Norwegian.

Examples:

• La være å røre den. = Don’t touch it.
• Kan du la være å si det? = Can you not say that?

In your sentence, ba meg la være å kaste pakkelappen means:

• asked me not to throw away the package label

Because pakkelappen is already in the definite form and means the package label.

The noun is:

• en pakkelapp = a package label
• pakkelappen = the package label

So kaste pakkelappen means throw away the package label.

Norwegian usually attaches definiteness to the end of the noun instead of using a separate word like English the.

Pakkelappen literally means the package label.

Depending on context, it could mean:

• a shipping label
• a parcel label
• a return label
• a label attached to a package for mailing

The exact English translation can vary a little depending on the postal situation, but package label or parcel label is a good general understanding.

Hadde fått is the past perfect, also called the pluperfect:

• fikk = got / received
• hadde fått = had gotten / had received

The past perfect is used because the sentence is looking back at one past action from another past point.

The sequence is roughly:

1. The sender asked me not to throw away the label.
2. They needed to receive it back first.

So Norwegian uses hadde fått to show that receiving it back had to happen before I threw it away.

In English, this is often translated with had received or had gotten.

In this sentence, de refers back to avsenderen.

That may feel surprising because avsenderen is singular (the sender) while de usually means they.

There are two likely reasons this happens:

1. De can be used as singular they when the sender’s gender is unknown or irrelevant.
2. Avsenderen might refer to a company, shop, or organization, and then they is also natural.

So here de hadde fått den means:

• before they had received it

Even though avsenderen is singular in form, de is still natural.

Den refers to pakkelappen.

Since pakkelapp is a common-gender noun (en pakkelapp), the pronoun used for it is den:

• en pakkelapp → den

So:

• før de hadde fått den i retur = before they had gotten it back

Here, it = the package label.

I retur means back, returned, or in return shipment depending on context.

In this sentence:

• få den i retur = get it back / receive it back as a return

This is a common phrase in postal, shipping, and customer-service contexts.

Examples:

• Pakken kom i retur. = The package came back / was returned.
• Vi fikk varen i retur. = We got the item back.

So hadde fått den i retur means had received it back.

Norwegian often uses på with places and institutions where English would usually say at.

So:

• på postkontoret = at the post office

This is just normal preposition usage in Norwegian. It does not literally mean on the post office in the English sense.

Other similar examples:

• på skolen = at school
• på sykehuset = at the hospital
• på kontoret = at the office

So på postkontoret is the normal way to say at the post office.

A simple breakdown is:

• Avsenderen = subject
• ba = main verb
• meg = object
• la være å kaste pakkelappen = what I was asked not to do
• før de hadde fått den i retur på postkontoret = time clause introduced by før

So the structure is:

• The sender asked me [not to throw away the package label] [before they had received it back at the post office].

The key thing to notice is that Norwegian keeps the time clause after the main statement, introduced by før = before.

Yes, it is natural and understandable.

A native speaker would understand it as something like:

• The sender asked me not to throw away the package label before they had received it back at the post office.

In smoother English, you might also phrase the idea as:

• The sender asked me not to throw away the parcel label until they had received it back from the post office.

So even if some parts do not map word-for-word onto English, the Norwegian sentence is built in a normal way:

• ba for asked
• la være å for not do
• hadde fått for had received
• i retur for back / returned