Thanks, Tommy. I just had a look at my purchases before the "mysterious 3x" era, trying to find merchants that classified as "Internet Purchases" in order to predict 3x bonuses. I suspect that these will work in addition to those I have confirmed before:
Amazon Kindle purchases
Amazon Payments*
Amazon Local
Amazon Video On Demand
Amazon Services Jersey (overseas)Apple webstore
apple.com
itunes.com
hulu.com
art.com
ononesoftware.com
onlineshoes.com
proclip USA
Oasis Date Gardens
worldpantry.com
ecohomestore.com
BigSkinnyCorp
shoes.com
Uncommongoods
Handicappedpets.com
invisibleshoe.com
Amazon Payments may be like Paypal, as it could depend on the merchant in question whether it classes as Internet Purchase. YMMV.
I was very pleased to see Hulu, Amazon kindle, iTunes and Apple there, and so will be adjusting my spending accordingly.