Sorry to bump an old thread, but I figured it wouldn't hurt anything.
Stamps.com is someone you definitely need to call and verify your cancellation with. I cancelled their shatty service well within the 2 week trial period, and was in turn billed month after month to my Paypal account (which is now closed because of this.) On hold with them now (going on about 15 minutes) to b1tch them out and demand refund and/or threaten chargebacks even if I can't charge it back with a closed account.