I tried this approach once and it turned out that they miscalculated and hence billed me for the wrong amount the first time around - an error that was caught during the final review process just before mailing the license out. That delayed the process considerably as they sat on the paperwork and contacted me to retrieve my billing details again, since the original details were shredded as soon as the payment was made. So leaving the form blank and deferring responsibility for the fee calculation to the CAA is no guarantee that you'll be able to avoid delays in the process - but that said, it's probably still a sensible approach for those who otherwise don't fully comprehend the scheme of charges.