У разі якщо шахрай заплатив карткою, оскаржте платіж.

У разі якщо means if, in the event that, or in case. It is more formal and more typical of instructions, official notices, banking language, and customer-support texts.

A simpler everyday version would be:

Якщо шахрай заплатив карткою, оскаржте платіж.

So the difference is mostly style and register, not basic meaning.

Because у разі якщо is being treated as one compound conjunction, meaning if / in the event that.

So the structure is:

У разі якщо шахрай заплатив карткою, ...

not:

У разі, якщо ...

You may sometimes see a comma inside similar expressions in other contexts, but here the normal reading is that у разі якщо functions as one unit.

Шахрай is in the nominative singular because it is the subject of заплатив.

It answers the question who paid?
→ шахрай

If it were an object, you might see another form, such as шахрая.

Заплатив is past tense, masculine, singular.

In Ukrainian, past-tense verbs agree with the subject in gender and number. Since шахрай is a masculine singular noun, the verb is masculine singular too:

• він заплатив = he paid
• вона заплатила = she paid
• вони заплатили = they paid

So шахрай заплатив literally has the grammar of the fraudster paid.

Because it expresses the means or instrument: by card, with a card.

After verbs like платити / заплатити, Ukrainian often uses the instrumental for the method or tool:

• платити карткою = to pay by card
• писати ручкою = to write with a pen

So карткою answers чим? = by what? with what?

Оскаржте is the imperative form of оскаржити, so it means dispute or challenge as a command/instruction.

The ending -те can mean either:

• a command to more than one person, or
• a polite/formal command to one person

That is why it is common in instructions addressed to the reader.

Compare:

• оскарж = dispute!
  informal singular
• оскаржте = dispute!
  plural or polite singular

Here платіж is in the accusative singular because it is the direct object of оскаржте:

оскаржте що? → платіж

For many masculine inanimate nouns in Ukrainian, the accusative looks the same as the nominative. So:

• nominative: платіж
• accusative: платіж

That is why the form does not change.

Because the sentence begins with a subordinate conditional clause, and then the main clause follows.

Structure:

У разі якщо шахрай заплатив карткою,
оскаржте платіж.

So the comma separates:

1. the if-clause
2. the main instruction

This is the same idea as English If X happened, do Y.

Because the sentence refers to completed, one-time actions.

• заплатив = paid, completed the payment
• оскаржте = dispute it, perform that action

Ukrainian aspect is very important. Here the perfective forms fit the meaning:

• first, the payment happened
• then, the reader should take one concrete action in response

If the sentence were about an ongoing or repeated process, imperfective forms might be used instead.

Yes. Ukrainian word order is flexible.

For example, you could also say:

Оскаржте платіж, якщо шахрай заплатив карткою.

The meaning stays basically the same. The original version puts the condition first, which is very natural in instructions.

Yes. That would still be correct:

Якщо шахрай заплатив карткою, оскаржте платіж.

This version sounds less official and more straightforward.
The original with У разі якщо sounds more formal, which fits banking or fraud-prevention language well.