wonder what would happen if I 'continue' without buying a bundle (or adding a seat or buying luggage checked baggage) at this stage, then after purchasing the ticket, attempt to add the bundle.
I've purchased add-ons in the manner before, but that was when I had been able to buy add-ons at initial booking but had chosen not to. I had bought meals and seats this way. Not sure if that will work when there is an issue like this present. AirAsia support didn't suggest it, but even if it's possible, I wouldn't expect them to suggest something this elaborate (probably asking too much of their scripted answer support service).
I remember when I had bought add-ons few days after the booking, Airasia didn't charge a separate credit/debit card fee when charging for the add-ons.