My customer agreement, like nearly everyone else's, say that there's a grace period for purchases, so if I pay my bill within 20 days after they send it, there's no interest charge. This seemed a little too good to be true, so I called them up and asked whether there was a grace period (yes), how much interest they would charge if I paid within the grace period (none), and just to confirm, she said there was a grace period just like a purchase with no interest due if I paid on time (yes.)

Well, in that case, that means they're free money, so I wrote myself a check for $10,000 and deposited it in my bank's high interest on-line savings account, which happens to be the account I can use to to pay the credit card bill on-line. (I can't deposit checks over the counter to the on-line account, but deposits in the ATM in the lobby are OK. Go figure.) I'll wait for the $10,000 to show up on the next bill, point 'n' click to pay the bill on the due date, leaving a month's interest in my account. High interest these days is only 1.35%, but that's still about ten bucks a month, and maybe this will get Capital One to stop sending me the blasted things.

The day after the deposit, the $10K showed up in my bank account. The day after that Capital One emailed me to tell me my balance transfer had been approved. That was a bit strange, but a quick look at their web site confirmed that the $10K had been posted as a purchase check, and so far no interest.

They should bill me on Dec 11th, so assuming the no-interest claims were true, I'll deposit the next $10K the day after that. Check back for updates then.