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Your statement shows a returned direct debit of 12 euros and, two lines further down, a bank charge of 35. The item is called a "debt-claim fee" — comisión por reclamación de posiciones deudoras — and it is not the same as overdraft interest, which you pay for the money the bank has fronted you: this is charged for the work of notifying you. Thirty-five euros to chase up twelve is nearly three times the bill itself, and you do not always have to pay it.
Before anything else, copy out the sentence you are going to write to the bank, because everything else hangs on it: "I request a refund of the 35-euro fee and ask the bank to evidence what individual, real and effective action it took to claim this outstanding balance, on what date, by what means and with what result." That is the question they find hard to answer.
The First Chamber of Spain's Supreme Court, the one that settles civil disputes, examined this fee in judgment 566/2019 of 25 October 2019 and declared the defendant bank's clause abusive because it was applied automatically, could be repeated on the same debt and did not correspond to any service actually rendered. The Bank of Spain, the supervisor of the Spanish banking system, had four conditions on the books, and that judgment sets them out: that it answers to an effective, specific and evidenced claim action; that it is not repeated on the same balance through successive steps with the same aim; that its amount is a fixed sum and never a percentage of the debt; and that it is not applied automatically. That it should also be written into your contract is a prior requirement, not one of the four. Its compendium of good practice has been widened since then — it now also looks at the information you were given before signing and at what the work actually cost — but what you have to obtain is the same: make the bank show what it did.
First, open the statement and find the exact name of the charge. Note the date, the amount, the bill that triggered it and whether that same debt produced more than one charge. If what appears is a plain overdraft fee, that is a different thing and this route does not apply.
Second, look at your account contract and check whether the fee was agreed and for what amount. Keep that page. Its being in the contract does not close the argument: the bank still has to show what specific action it took, distinct from the automatic entry.
Third, file a written complaint with your bank's customer service department, through a channel that gives you proof of receipt: an online form, a registered email address or the branch's register. Identify the account, the date and the amount, paste in the sentence above and also ask whether the fee was generated automatically and whether another had already been charged on the same debt. The bank has fifteen working days to reply when the complaint concerns a payment service — and a current account with direct debits is one — and one month in other cases if you are a consumer.
Fourth, if they say no or the deadline passes with no reply, take the same paperwork to the Bank of Spain's Conduct of Entities Department, which reviews whether the bank complied with transparency rules and good banking practice. Only a judge can declare a clause abusive, so frame it around what the bank did or failed to do, not around the clause itself. It is free, it is done online, and you have to attach the earlier complaint, its receipt, the reply if there was one, the statement and the contract clause. Keep a copy of everything until the end.
The report the Bank of Spain issues does not oblige the bank to give you anything back: it says who is right, and most banks correct the charge, but if yours refuses, only the courts are left. Up to 2,000 euros you can appear yourself, with no lawyer and no court agent.