W aptece proszę o maść i plaster.

Because w meaning in or inside takes the locative case when it refers to location.

• basic form: apteka = pharmacy
• after w for location: w aptece = in the pharmacy / at the pharmacy

So apteka changes to aptece because of case. The k → c change is a normal Polish spelling/sound alternation in some forms.

Grammatically, proszę is the 1st person singular form of prosić, so literally it means I ask or I am asking.

But in real-life Polish, proszę is also very common in polite situations and can feel like:

• please
• I’d like
• may I have

So in this sentence, it is not just a dry literal I ask. It has a polite request meaning.

Because the verb pattern is:

prosić o + accusative = to ask for

So:

• proszę o maść = I’m asking for ointment
• proszę o plaster = I’m asking for a plaster/bandage

This is worth memorizing as a fixed pattern.
Even though o often means about, after prosić it means for.

They are in the accusative, because prosić o takes the accusative.

In this sentence, though, the forms do not visibly change:

• maść → maść
• plaster → plaster

That happens because:

• maść has the same nominative and accusative singular form
• plaster is a masculine inanimate noun, and those often have the same nominative and accusative singular form

So the case is there grammatically, even if the spelling stays the same.

A noun that does change would be, for example:

• tabletka → proszę o tabletkę

Because Polish has no articles.

So Polish does not have separate words corresponding directly to English a/an and the.
Whether something is specific or general is usually understood from:

• context
• word order
• intonation
• extra words if needed

So maść i plaster can mean something like:

• ointment and a plaster
• the ointment and the plaster
• some ointment and a plaster

depending on the situation.

Yes, it is grammatical and understandable.

That said, in everyday spoken Polish, many people would more naturally say:

• Poproszę maść i plaster.

This often sounds more normal for ordering or asking for items at a shop, pharmacy, or counter.

So:

• Proszę o maść i plaster. = correct, polite, slightly more formal/bookish
• Poproszę maść i plaster. = very common in real life

Both can be polite, but they are used a little differently.

• proszę comes from prosić and means I ask / please
• poproszę comes from the perfective verb and is very common for a single, concrete request

In service situations, poproszę often sounds especially natural, like:

• I’ll have...
• I’d like...
• May I have...

So at a pharmacy counter, Poproszę maść i plaster is probably the most natural everyday version.

Not always exactly.

In a medical context, Polish plaster usually means:

• adhesive plaster
• sticking plaster
• often what American English calls a Band-Aid

So if you are a native English speaker, be careful: Polish plaster is usually not construction plaster here.

Also, if you mean a bandage wrap, Polish more often uses:

• bandaż

So:

• plaster = adhesive plaster / bandage strip
• bandaż = bandage roll/wrap

Roughly:

• proszę ≈ PRO-sheh
• aptece ≈ ap-TE-tseh
• maść ≈ something like mahshch, said very quickly as one syllable

A few useful sound notes:

• sz sounds like English sh
• ś is a softer sh
• ć is a softer ch
• final ę is often pronounced more simply, especially in everyday speech

English-style spellings are only approximations, but they can help at first.

Yes. Polish word order is quite flexible.

W aptece proszę o maść i plaster is understandable, with w aptece setting the scene first: at the pharmacy.

But if you are actually standing in the pharmacy, you would usually just say:

• Poproszę maść i plaster.

You can move words around for emphasis, but not every order sounds equally natural in every context.
For a learner, the safest natural options are:

• Proszę o maść i plaster.
• Poproszę maść i plaster.

If the place is already obvious, w aptece is usually unnecessary.