Used my voucher today, chatbot useless, so telephoned. The price of new flight exceeded voucher, just as I was about to get out my CC I remembered something about having booster value, telephone rep couldn't see anything on his system initially, I was able to pull up the email on my laptop and give details. Rep then put me on hold, after a minute or so he came back on and agreed that there would be no excess fee.
Posting as a word to the wise, have your email trail in front of you in case the rep doesn't proactively take any boost into account.
Other than this overlooked matter, I couldn't fault the assistance and service.