Hi @BrendonLV, welcome!
The sign-up cost will be charged on your regular monthly bill, not separately. You can check your billing through your MijnKPN personal environment. You login there using your KPN Account.
Once logged in, you can see your bills. And also change your bank account details by going to Producten at the top, clicking your subscription and then choosing ‘Persoons- en betalingsgegevens’. On the page that loads next, click on ‘Wijzigen’ in the bottom block to change your bank account details.
You can also use the MijnKPN app for this. There, you choose the Profile option and then tap ‘Persoons- en betalingsgegevens’. On the next page, choose your subscription, and then tap ‘Aanpassen’ in the bottom block.
If you don't have a KPN Account yet, use the right side block on the MijnKPN log-in page to create your account. Then, after doing so, you can login and attach your subscription to it. This can also be done with the MijnKPN app. After doing this, follow the above mentioned steps.