If your account balance shows that you have paid, but the online retailer says it has not received payment, the online retailer should contact its own bank. To do this, it needs the following details:
All of this information together can be found in the description field for the relevant payment. The online retailer can use this information to make enquiries at its bank.
If you are not successful in solving this yourself with the online retailer, contact your own bank. It is then up to your bank to provide further assistance in identifying where the error occurred. It is also important to have the 16-figure iDEAL transaction number to hand for this purpose.