Costco plan deal, add another line, would price be different from what i predict?

Sorry, still not enough information. I don't know if this is a special deal through Costco. 

Have no idea whether you qualified for a 25% discount for being a doctor, nurse, teacher, first responder. 

I have no idea if adding a third line would muck up your deal. I have no idea whether you can even add a third line 

And I'm not sure I'd trust AT&T to answer the question honestly. Maybe talk to Costco again 

Hi @apersonwholivesintheworld

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Just as a heads up, the AT&T kiosks in places like Costco are not owned or operated by AT&T nor are they staffed with AT&T employees so be very careful about any plans or offers made by them because AT&T will only honor what is listed, in detail, on their website. So if you have any issues, you did business with the independent, not AT&T. Make sure you carefully read and understand the TOS when it comes to billing, canceling, etc. Any deal that is not listed on AT&T's website is Costco's deal so you will need to work with them.