The specs you posted are wrong--congrats, you have a much nicer bike! Carbon frame and Ultegra components.
Carbon fiber frames are not well designed to resist clamping forces or localized forces tangential to the frame tubes. Basically, don't put a rack on the thing. The only vaguely sane idea is a rack that mounts to the brake bridge and the QR skewer, like the Axiom already mentioned or the Tubus Fly. Unless the grocery store is far away I'd suggest wearing a backpack. 30 lbs is fine for a mile or two. For what its worth I do most of my grocery shopping with a 25 liter backpack on a road bike with no rack. While panniers carry heavy loads very well, the convenience of into the store a handful of blocks away with a backpack is also pretty fantastic.
If you want some carry options, I'd get a large bikepacking saddlebag and frame bag. Not a worthwhile investment for occasional groceries, but maybe worth it if you're needing to carry spare clothes, a laptop, etc to and from work.