Если Вы не знаете почтовый индекс, посмотрите его на сайте почты.

Вы is the polite/formal way to say you in Russian. It can also mean you plural, but in this kind of instruction it is usually formal singular or formal address to the reader.

The capital letter is a politeness convention in formal writing, especially in letters, notices, customer communication, and instructions. In many modern texts you will also see lowercase вы even when it is polite, so the capitalization is respectful rather than grammatically required.

Если means if and introduces a conditional clause.

So the structure is:

Если Вы не знаете почтовый индекс, посмотрите его на сайте почты.
= If you do not know the postal code, look it up on the postal service’s website.

This is a very common Russian pattern: если + statement, then the main clause.

In Russian, не is usually written separately from finite verb forms.

So:

• знаете = you know
• не знаете = you do not know

This is normal verb negation in Russian.