Yes, that would be theft.
It's anyone's prediction whether a prosecutor would go after you, and whether a judge would buy it.
My personal prediction is you're safe in that case, since (i) it does sometimes happen that they lower prices to way below cost to drive up traffic, and sometimes without advertising, and (ii) at the end of the day, they can choose to just not honor it, which is what usually happens in that case anyway.
I did order like 10 infant life jackets the other week, and I have zero concern about it (and they were all cancelled, and I didn't complain).
But sometimes judge and prosecutors act unpredictably. Sometimes they sentence good people to jail for 27 years for a loss that the prosecutors themselves created and should have avoided.