I think you mistook my post, I would recommend using exact for BOTH the positive and negative keywords.
In your case, I believe, the positive keywords being exact would solve all your problems. One, make sure these signups are coming from PPC, then for example, why not add [airasia payment online by debit] to your keyword list. You've already told me it sells and you would be able to track the metrics for this keyword exactly. Then, take the list that goes on and on, and export it to an excel sheet, add those keywords to your PPC campaigns. Make sure you put the [] around each though. Excel can help you do this very quickly if you are familiar with excel functions. Just my $.02. Hope it helps.
Trenton