Interesting. Whenever I make changes that affect how much I'll be billed, the amounts get added or removed and take effect when the card next gets charged (which happens once a month). At least that's how I remember it... going to dig through some old statements now to verify this as its been a while since I last changed anything that would alter what they charge me.
I had to go back further than I thought to find an off-cycle charge, but I did find one from January 2012, for a plan upgrade, 10 minutes before midnight, for $6.77. It looks like that was the last plan modification I made.
They may've changed their billing system in the last 12+ months.
I haven't got online statements going back far enough to check right now (will have to do it with paper copies when I get home if I can be bothered). You're probably right though :) I guess they either keep the passphrase in memory (someone enters it once on boot) or another possibility is they have a couple of billing windows each day where they batch through whatever charges/refunds have accumulated since the last run. Feasible I guess with 2 or 3 specially cleared staff members on rotation.
