Newel How are you trying to pay? SEPA direct debit, Mastercard, Sofort or iDeal? Maybe there is another payment method you can try. In general you should probably try to ask the merchant if there is a problem with their payment process.
Edit: Seems like Laposte accepts both MasterCard and PayPal. Both should be usable with your bunq account. Did you try that? And if yes, what exactly went wrong? Is there any error message on the website or the bunq app? Without details it's hard to give any advice.