I'm not gonna go to the grocery store even after I said I would.
Why wouldn't you do what your promised?
I'm feeling lazy and don't wanna do it.
If you told someone that you were going to the store for them you should follow through on it. They are expecting you to buy those items and they may need them.
If they really needed them so bad, they should have gone themselves instead of asking me.
Regardless of if you think they should have gone instead, you promised to go. By breaking that promise you are putting them in a difficult spot and also making them less likely to trust you in the future. You should just go to the store for them and stick to your word.