Zapakuję przesyłkę zwrotną pod warunkiem, że oryginalne opakowanie będzie suche.

Because zapakuję comes from the perfective verb zapakować.

In Polish:

• imperfective verbs can form a present tense and a compound future
  
  • pakuję = I am packing / I pack
  • będę pakować = I will be packing
• perfective verbs do not normally have a true present meaning; their “present” forms refer to the future
  • zapakuję = I will pack

So zapakuję is grammatically a present-form ending, but in meaning it is future, because the verb is perfective.

Also, the ending -ę shows 1st person singular: I.

Because this phrase is the direct object of zapakuję, so it must be in the accusative case.

Base form:

• przesyłka zwrotna = return shipment / return parcel

Accusative singular feminine:

• przesyłka → przesyłkę
• zwrotna → zwrotną

So:

• Zapakuję przesyłkę zwrotną = I will pack the return shipment

This is a very common pattern with feminine nouns ending in -a:

• nominative -a
• accusative -ę

And the adjective changes to match the noun.

Here zwrotną means return in the sense of being sent back.

So przesyłka zwrotna means something like:

• return shipment
• return parcel
• item being returned

It does not mean reflexive here, even though in grammar-related contexts zwrotny can be connected with reflexive ideas. In this sentence, it is about a shipment going back.

Pod warunkiem, że is a fixed expression meaning:

• provided that
• on condition that
• as long as

So:

• Zapakuję przesyłkę zwrotną pod warunkiem, że... = I will pack the return shipment provided that...

It introduces a condition that must be met.

This is a very useful phrase to memorize as a whole.

Because warunkiem is the instrumental singular form of warunek.

• warunek = condition
• warunkiem = instrumental form

In the fixed expression pod warunkiem, Polish uses this instrumental form. Literally, the phrase is something like under the condition, but in natural English it means provided that.

For learners, the easiest approach is to memorize:

• pod warunkiem, że = provided that

without trying to build it from scratch each time.

Because the sentence is talking about a future condition.

• będzie = will be
• jest = is

The meaning is:

• I will pack the return shipment, provided that the original packaging will be/is dry at that future time

In natural English, we often say provided that it is dry, but Polish commonly uses the future here when the condition refers to a future situation:

• oryginalne opakowanie będzie suche

So będzie is completely natural.

Because they agree with opakowanie, which is a neuter singular noun.

Base noun:

• opakowanie = packaging / package / box / wrapping

Since it is neuter, adjectives describing it must also be neuter singular:

• oryginalne opakowanie = original packaging
• opakowanie będzie suche = the packaging will be dry

So:

• oryginalny = masculine
• oryginalna = feminine
• oryginalne = neuter

and

• suchy = masculine
• sucha = feminine
• suche = neuter

After być (to be), the adjective here stays in the nominative and agrees with the subject.

So:

• opakowanie = neuter singular nominative
• suche = neuter singular nominative adjective

That is why:

• opakowanie będzie suche

This is similar to simple equational or descriptive sentences:

• Dom jest duży = The house is big
• Dziecko będzie zmęczone = The child will be tired

The adjective matches the subject.

They are different things:

• przesyłka = the shipment / parcel / item being sent
• opakowanie = the packaging / box / container / wrapping

So the sentence is saying that the speaker will pack the return shipment, but only if the original packaging is dry.

In other words, one noun refers to what is being sent, and the other refers to what it is packed in.

Because in Polish, a clause introduced by że normally takes a comma.

So:

• pod warunkiem, że oryginalne opakowanie będzie suche

needs the comma before że.

This is a very standard Polish punctuation rule. If you see że introducing a subordinate clause, there is usually a comma before it.

Yes. Polish word order is fairly flexible.

For example, you could also say:

• Pod warunkiem, że oryginalne opakowanie będzie suche, zapakuję przesyłkę zwrotną.

This version puts the condition first, so it sounds a bit more like:

• Provided that the original packaging is dry, I will pack the return shipment.

The original sentence is perfectly natural, but changing the order can shift emphasis.

Yes, but the meaning changes slightly.

• zapakuję suggests a completed action: I will pack it
• będę pakować focuses more on the process: I will be packing / I will pack in a less result-focused way

In this sentence, zapakuję is more natural because the speaker is talking about a concrete result: the return shipment will get packed, if the condition is met.

So the perfective verb fits well.