Tant si el venedor respon com si no, reclama el reemborsament.

It means whether ... or not or regardless of whether ....

So:

Tant si el venedor respon com si no, reclama el reemborsament.
= Whether the seller replies or not, claim the refund.

This is a fixed Catalan pattern:

• tant si X com si no = whether X or not
• tant si X com si Y = whether X or Y

So it is best learned as a chunk, not translated word by word every time.

Not really in any useful literal way. In this sentence, tant si is part of a set expression.

A learner may know tant from sentences like:

• Tinc tant de temps = I have so much time

But in tant si ... com si no, you should treat the whole pattern as a connector meaning whether ... or not.

Respon is the 3rd person singular present indicative of respondre.

So:

• el venedor respon = the seller replies / responds

It matches the subject el venedor.

A native English speaker may expect something more obviously future-looking, but Catalan often uses the present for open future situations, especially in structures with si.

Because this sentence is talking about a real alternative: the seller may reply, or may not reply.

In the fixed structure tant si ... com si no, Catalan normally uses the indicative for this kind of neutral, real possibility.

This is easy to confuse with another use of com si, meaning as if, which does usually trigger the subjunctive:

• Parla com si ho sabés = He speaks as if he knew it

But that is a different construction.
Here, com si no is part of tant si ... com si no, not the as if pattern.

Because Catalan often leaves out repeated words when they are obvious from context.

So:

• tant si el venedor respon com si no

means essentially:

• tant si el venedor respon com si no respon

The second verb is understood, so it does not need to be repeated.

English does the same thing:

• whether the seller responds or not

You do not need to repeat responds after or not either.

Here it is a command: claim / request the refund.

So reclama is the imperative for tu.

It is true that the form reclama can also look like the 3rd person singular present indicative:

• Ell reclama... = He claims...

But in this sentence, the structure and punctuation make the imperative reading the natural one:

• first comes a subordinate clause
• then, after the comma, the main instruction

So the meaning is Whether the seller replies or not, claim the refund.

It is simply omitted.

Catalan usually drops subject pronouns when the verb form already makes the person clear. This is very common.

So:

• reclama = claim / you claim in command form

The understood subject is tu.

If you add the pronoun, it is usually for emphasis:

• Tu reclama el reemborsament

But the neutral, normal version leaves tu out.

Because reclamar is stronger than demanar.

• demanar = to ask for
• reclamar = to claim, often with the sense that you have a right to it

In a consumer-rights context, reclamar el reemborsament sounds very natural, because the idea is not just politely asking, but insisting on the refund you are entitled to.

A few rough differences:

• demanar un reemborsament = ask for a refund
• reclamar el reemborsament = claim the refund
• sol·licitar un reemborsament = request a refund, more formal/administrative

Because this refers to a specific refund: the refund connected with the purchase or dispute already being discussed.

So:

• el reemborsament = the refund
• un reemborsament = a refund

Using el makes it sound like there is one definite refund that should be given.

Because the first part of the sentence is an introductory subordinate clause:

• Tant si el venedor respon com si no, ...

Then the main clause follows:

• reclama el reemborsament

The comma helps separate those two parts clearly. It works much like English punctuation in a sentence such as:

• Whether the seller replies or not, claim the refund.

Yes.

If the subject is already clear, Catalan can shorten the first part:

• Tant si respon com si no, reclama el reemborsament.

You can also move the main clause first:

• Reclama el reemborsament, tant si el venedor respon com si no.

Both are grammatical. The original version is just very clear and natural, especially in instructions.