At my LFS, they have separate O2 and CO2 valves that they use for fish and plants, respectively. This may be the case with you, too. Maybe not, and I'm being an ***.
Anyway, plants can and do respire (take in oxygen and release carbon dioxide). They do it at night, or whenever there's not enough light for them to photosyntehsize. That's why it's often recommended to turn on aeration at night, to avoid both your plants and your fish from suffocating.
If you plan to take your plants home within an hour or so, I'd say don't worry about it. If you are still worried, just open the bag for them.