What should a girl do who took a piece of clothing from a store without the owner's knowledge and is afraid to go back to him, and how can the item be compensated for with fluctuating prices?

It is not permissible to replace an item from a store without their knowledge, so your taking the item without right is considered invalid. Allah Almighty said: ﴿O you who have believed, do not consume one another’s property unlawfully but only [in lawful] trade by mutual consent.﴾ The Prophet, peace and blessings be upon him, said: (The property of a Muslim is not lawful except with his willing consent).

You are obliged to repent and return the item to its owners, for the Prophet, peace and blessings be upon him, said: (On the hand is what it took until it fulfills it). You must inform the store owners of what happened, and they have the choice to forgive you, demand its price, or retrieve the item. If shyness prevents you, then have someone else do it on your behalf.