I'm going to be on my phone while making purchases at a store
What do you plan on buying?
I'm going to buy an iPad because people like me can afford it and I'm sorry if it bothers you.
Just understand that the cashier could find it upsetting for you to be on your phone while talking with them. If your intent is not to be rude than it may be better that you not do this.
I'm not a rude person, why are you calling me rude?
Absolutely not. Just letting your know that the other person may perceive you as rude if you do this.