PDFaid Unauthorized Charge After One Time PDF Payment

I am writing to contest a debit of €39.00 that appeared on my bank account today at 14:27. This charge comes from the site PDFaid.com, where I had previously made a single payment of €0.50 at the end of December for a simple PDF conversion service, without subscribing to any form of subscription or recurring payment. I never authorised any additional debit beyond that initial €0.50 payment, nor did I validate any subscription. On the transaction, it is indicated that the payment was first refused and then later accepted, yet I never received any notification or request for confirmation from your side, and I never gave my consent for this payment to be accepted. I therefore formally contest this €39.00 debit and ask you to verify the origin and justification of this transaction and to inform me of the steps to follow in order to obtain a refund, if this debit is not legitimately due. I have already contacted my bank regarding this transaction but have not yet received any response. I kindly request that you review this situation as soon as possible and provide me with a clear explanation of this charge and a solution, including a refund of the €39.00 where appropriate.