Learn Russian: Доплата за багаж составит рублей пятьсот. - The surcharge for luggage will amount to about five hundred rubles.

In Russian, you can swap the order of a number and its noun to mean approximately or about. While пятьсот рублей means exactly five hundred rubles, putting the noun first as рублей пятьсот means about five hundred rubles.

Does swapping the word order change the cases of the words?

No, the grammatical relationship stays exactly the same. The number пятьсот still forces the noun into the genitive plural, which is why we use рублей. The only thing that changes is the word order to show approximation.

Why do we use the accusative case for за багаж?

The preposition за is used with the accusative case when talking about paying, charging, or compensating for something. Since багаж is an inanimate masculine noun, its accusative form looks exactly like its nominative form.

What is the infinitive of составит and why is it perfective here?

It is the future tense of the perfective verb составить which means to amount to or to make up. It is perfective because it refers to a specific, one-time final amount that the surcharge will reach.