Ich schaue mir das Sortiment an, bevor ich zur Kasse gehe.

Because anschauen is a separable-prefix verb (an- + schauen). In a main clause, the conjugated verb goes in position 2 and the separable prefix goes to the end:

• Ich schaue … an. In an infinitive or subordinate clause, it stays together (e.g., …, weil ich mir das Sortiment anschaue.).

sich etwas anschauen commonly takes:

• a dative reflexive pronoun (mir/dir/sich/uns/euch) and
• a direct object (here das Sortiment, accusative). So it’s literally like “I look at the assortment for myself / “I take a look at the assortment.” Using mich would change the structure and is not the normal pattern for this verb in this meaning.

It’s accusative, because it’s the direct object of (sich) anschauen (“to look at something”). You can tell because:

• The article das could be nominative or accusative, but here it’s not the subject (Ich is).
• So das Sortiment must be the object → accusative.

Because bevor introduces a subordinate clause. In German, subordinate clauses are separated by a comma:

• Ich schaue mir das Sortiment an, bevor …

In subordinate clauses (introduced by words like bevor, weil, dass, wenn), German typically sends the conjugated verb to the end:

• …, bevor ich zur Kasse gehe. Main clause: verb in position 2. Subordinate clause: verb-final.

zur is a contraction of zu der:

• zu + der → zur It’s extremely common in everyday German. You’d only keep zu der uncontracted for strong emphasis or contrast.

The preposition zu always takes the dative. Since Kasse is feminine, dative singular is der Kasse, and with contraction:

• zu der Kasse → zur Kasse

German normally requires an explicit subject in each clause, so ich is repeated:

• …, bevor ich … gehe. You generally can’t omit it the way you might in some other languages. You can rearrange the sentence, but the subject stays:
• Bevor ich zur Kasse gehe, schaue ich mir das Sortiment an.