Thanks all; much appreciated. I've never had any kind of coupon problems at my CVS, so this one surprised me.

I'm pretty careful to make sure my coupons are processed before the ECBs, and I don't think I gave the ECBs to her before the coupons. The cashier wasn't mean or rude about it - she was just as confused as I was. We tried re-ringing things in different ways to see if there was some bizarre error going on, but the error message only showed up on the register when she tried ringing the IP coupons. I don't mind doing two transactions, but it just surprised me since my local CVS is so coupon-friendly.

edit: had to clarify a sentence that made no sense; I'm clearly hungover from my super-doubles HT trip this morning.