Margen
1. Compare the first price with the last one. You choose the seat on a screen, you download the ticket yourself and even so the final summary carries a line that was not on the poster. Take a screenshot when you pick the seat and another just before paying. If the poster says 30 euros and the final summary says 34.50, those 4.50 are the 15% that was not advertised. Royal Legislative Decree 1/2007, the Spanish consumer law, requires in its article 60 that you be told the total price, taxes and additional charges included, before you are bound by the purchase. Nobody refunds you a charge you accepted without seeing it.
2. Ask for the same seat at the venue's box office. Same section, same performance, same day. Do not assume the website is cheaper, or that the box office escapes every charge: compare. And compare the breakdown, not just the total, because two prices can differ because of a promotion, an allocation or different conditions; only a line identified as a commission tells you how much the sales channel is taking.
3. Check every box before confirming. Article 60 bis of that same law prohibits charging you for additional payments you have not expressly accepted: a cancellation insurance or any other extra cannot come pre-ticked and count your failure to click as consent. If an option appears switched on, switch it off, screenshot it and check that the total goes down.
4. See whether they charge you for paying. That surcharge simply does not exist: Royal Decree-Law 19/2018, on payment services, forbids charging you anything extra for using a consumer card, a transfer or a direct debit. If that line turns up in the breakdown, do not negotiate it: claim it in writing.
5. Check what handling you actually received. See whether they send you a ticket for your phone, whether they require you to print it or whether they charge you to download it again. Keep the whole breakdown until the day of the concert, which is when it is useful for a complaint.
You choose the seat, you download the ticket and you provide the paper, the ink and the phone. It would only be fair for the breakdown to read "handling charges: yours".