In literary Ukrainian, when you use чи, you normally move the main verb to the beginning of the sentence: Чи шукаєте ви пошту?

Чи Ви шукаєте пошту could be understood 'Is it you who are looking for the post office?'

Accepted now

ні це не натурально звучить так само як сказати- Ви шукаєте почту кожен день

Yeah... It's not a common sentence to say. Nevertheless, it's grammatically correct and can be meaningful given a context. E.g. "How do you send a letter? - You drop it into a mailbox. - And how do you find a mailbox? Do you look for a post office? Or are there mailboxes in other places?"

So, now it's accepted.

Because that's not a word in Ukrainian.

Пошту is Accusative, пошті is Locative.

(На) пошті is used when asking "Where at?" (Я на пошті). Пошту is used when it's a direct object (Я шукаю пошту, "пошта" is a direct object of "шукаю") or when asking "Where to?" (Я йду на пошту).