Po wyjściu z urzędu kupuję kopertę i znaczek na poczcie.

Here po means after.

So Po wyjściu z urzędu means After leaving the office or more literally After the leaving from the office.

In this time-related meaning, po is followed by the locative case.

Because wyjściu is not a verb infinitive. It is a verbal noun formed from wyjść (to go out / to leave).

• wyjść = to leave
• wyjście = leaving / exit
• po wyjściu = after leaving

Polish often uses this structure:

• po + verbal noun in the locative

For example:

• po powrocie = after returning
• po przyjeździe = after arriving
• po wyjściu = after leaving

So this is a very normal Polish pattern.

Because z here means from / out of, and in that meaning it takes the genitive case.

The noun is:

• urząd = office, usually a government/administrative office

Its genitive singular form is:

• urzędu

So:

• z urzędu = from the office / out of the office

Compare:

• w urzędzie = in the office
• z urzędu = from the office

Urząd usually means an official office or government office, not just any office where people work.

Depending on context, it might mean things like:

• city office
• tax office
• administrative office
• registry office

So a learner should not automatically think of a private company office. It usually suggests some kind of public institution.

This is about aspect.

• kupować = imperfective = to buy, to be buying, to buy regularly
• kupić = perfective = to buy, to buy successfully/once, to end up buying

So:

• kupuję = I buy / I am buying
• kupię = I will buy / I’ll buy

In the sentence kupuję can sound like:

• a present-time statement
• a habitual action
• a present-tense narration

If the meaning is specifically After leaving the office, I will buy..., then kupię would often be more natural.

So the choice depends on the intended meaning:

• kupuję = present or habitual
• kupię = future one-time completed action

Because Polish often omits subject pronouns when they are clear from the verb ending.

• kupuję already tells you the subject is I
• the ending -ę marks 1st person singular

So ja is usually unnecessary.

You would add ja only for emphasis or contrast, for example:

• Ja kupuję kopertę, a on znaczek. = I’m buying an envelope, and he’s buying a stamp.

Because both nouns are in the accusative case as direct objects of kupuję, but they change differently.

• koperta is a feminine noun

• feminine nouns ending in -a usually change to -ę in the accusative singular
  So: koperta → kopertę

• znaczek is a masculine inanimate noun
• masculine inanimate nouns usually have the same form in nominative and accusative singular
  So: znaczek → znaczek

That is why you get:

• kupuję kopertę i znaczek

They are both in the accusative singular because they are the things being bought.

The verb kupować / kupić takes a direct object in the accusative.

So:

• kupuję co? = what am I buying?
• kopertę i znaczek

This is one of the most common uses of the accusative in Polish.

Here znaczek means a postage stamp.

By itself, znaczek can sometimes mean other kinds of small signs or badges depending on context, but with na poczcie the meaning is clearly stamp.

So:

• koperta = envelope
• znaczek = stamp

Because Polish idiomatically says na poczcie for at the post office.

Even though English uses in or at, Polish often uses na with certain places and institutions.

Here:

• na poczcie = at the post office
• na takes the locative when talking about location
• poczta → poczcie in the locative singular

Compare:

• Idę na pocztę. = I’m going to the post office.
  (na
  • accusative, movement)
• Jestem na poczcie. = I’m at the post office.
  (na
  • locative, location)

Yes. Polish word order is fairly flexible.

The sentence as given is neutral and natural:

• Po wyjściu z urzędu kupuję kopertę i znaczek na poczcie.

But you could also say:

• Na poczcie kupuję kopertę i znaczek po wyjściu z urzędu.
• Po wyjściu z urzędu na poczcie kupuję kopertę i znaczek.

These versions may sound less neutral or put emphasis on different parts.

In general, the original order is good because it presents the information clearly:

1. when
2. what happens
3. where

They are similar, but not exactly identical.

• po wyjściu z urzędu = after leaving the office
• kiedy wyjdę z urzędu = when I leave the office / when I have left the office

The first is a noun-based structure and often sounds compact and natural in Polish. The second uses a full verb clause and can sound a little more explicit.

Both are correct, but po wyjściu z urzędu is a very common Polish way to express after doing something.

No. Polish does not have articles like a, an, or the.

So:

• kopertę can mean an envelope or the envelope
• znaczek can mean a stamp or the stamp

You understand which one is meant from context.

In this sentence, the most natural English translation is usually:

• I buy an envelope and a stamp at the post office.

As written with kupuję, it can suggest either:

• a habitual action: After leaving the office, I buy an envelope and a stamp at the post office
• a present-time description/narration: After leaving the office, I’m buying an envelope and a stamp at the post office

If you want a clearly one-time future action, Polish would more often use:

• Po wyjściu z urzędu kupię kopertę i znaczek na poczcie.

So the exact interpretation depends on context, especially because of the aspect choice.