Congrats Kage. You should be fine. If you sold yourself during the Interview and gave them the impression that they need you more than you need them your fine. Just be up front with them from the start and explain to them the situation, and you should be ok.
But also keep note, I'm not sure if they have a probationary period, Given they do, usually there is a no tolerance for time off/tardiness. Keep in mind what I said as well, if they really took a liking to you, they very well may let it slide.
Congratulations on the new job man , Sorry to say that I think I make more than you do an hour, O well what is the Costco's, Like what do they do or produce?