I don't get why any store would do this kind of guarantee. How can you guarantee something that's out of your control?
That said, expecting a package from any store at any time within 1 day is kinda unrealistic unless you live near the warehouse. Even then, usually it takes 1-2 days just to process. I usually expect a week and if I get it within then I'm happy. I have seen the odd occasion where I get something the next day, but I must of gotten very lucky where the timing was right or something.