Most of the time if you get asked to go to another store on short notice it is due to a callout and because of that you don’t typically get scheduled ahead of time for those shifts. Unless it’s a borrowed partner type of thing. But that being said, you are completely okay declining those shifts. It’s not your responsibility and that work / life balance is hard to maintain when you’re being asked to come in so much. I struggling declining those shifts when my manager texts me but what I’ve gotten in the habit of saying is “Sorry, I can’t today. Thank you for thinking of me though!” or something like that

Next time they ask say : I’m so sorry but I have other commitments on my days off, or I’m sorry I have another commitment today

you can reply with "sorry but I won't be able to cover the shift." short and simple. its cool if you want the money but you should also enjoy your days off and it shouldn't feel like your responsibility to cover another store's short- staffed shift.