Κράτα τη συσκευασία και την απόδειξη, σε περίπτωση που θέλεις να το επιστρέψεις, αρκεί να είναι όλα μέσα.

Κράτα is the imperative (command) form of the verb κρατάω / κρατώ (to keep/hold), addressed to one person informally (2nd person singular).
So it’s like saying Keep … to a friend/customer in an informal tone.
More formal/plural would be Κρατήστε.

Both are forms of the feminine accusative article την (the, feminine, object form).
In everyday Greek, την often shortens to τη before a consonant sound:

• τη συσκευασία (s- is a consonant)
• την απόδειξη (a- is a vowel, so it usually stays την)

This is mainly about pronunciation flow.

• η συσκευασία = the packaging (feminine)
• η απόδειξη = the receipt / proof (feminine)

Because they’re feminine, they take τη/την in the accusative as direct objects of Κράτα.

The commas separate an inserted clause:

• Main command: Κράτα τη συσκευασία και την απόδειξη
• Extra info: σε περίπτωση που θέλεις να το επιστρέψεις
• Condition/requirement: αρκεί να είναι όλα μέσα

You can read it as: Keep the packaging and the receipt — in case you want to return it — provided everything is inside.

σε περίπτωση που means in case (that). It’s a fixed expression.
Greek often uses που here (literally that/which) rather than αν:

• σε περίπτωση που… = in case…
• αν… = if… (more direct conditional)

Both can sometimes translate as if, but σε περίπτωση που sounds more like “as a precaution”.

Greek commonly expresses want to + verb as:

• θέλω + να + verb

So θέλεις να το επιστρέψεις literally is you want that you return it, i.e. you want to return it.

το is a neuter object pronoun meaning it (the item/product).
In Greek, weak object pronouns typically come before the verb:

• να το επιστρέψεις = to return it

Even if the noun isn’t stated, το points back to the thing you bought.

After να, Greek uses the subjunctive. Here it’s the aorist subjunctive:

• (να) επιστρέψεις from επιστρέφω (to return)

Aorist subjunctive often views the action as a single complete event: to return it (once).
να επιστρέφεις would be more like to be returning (habitually/ongoing), which doesn’t fit as well here.

αρκεί means it is enough / it suffices and it’s commonly used as:

• αρκεί να + subjunctive = as long as / provided that

So αρκεί να είναι όλα μέσα = as long as everything is inside.

After αρκεί να, Greek again uses να + verb (subjunctive).
να είναι is the subjunctive form using the present stem of είμαι (to be). It expresses a condition/state that must be true: everything must be inside.

όλα is neuter plural meaning everything / all items. Neuter plural is often used for “everything” in general.
μέσα means inside (adverb).
So να είναι όλα μέσα means everything must be inside—i.e., all parts/accessories/documents are included in the package.

As written, it’s informal singular because of Κράτα and θέλεις.
More formal/polite (or plural) would be:

• Κρατήστε τη συσκευασία και την απόδειξη, σε περίπτωση που θέλετε να το επιστρέψετε, αρκεί να είναι όλα μέσα.