I'm actually on AT&T post-paid so the employee advised me to buy the phone and one month's service for $245+taxes and then put my post-paid SIM in the iPhone SE. She assured me that that the device would work fine on my post-paid plan and I would not be charged a cancellation or recurring fee of any kind for the GoPhone pre-paid plan.
I asked for a flyer or advertisement but she said it was an unadvertised promotion. She told me she wasn't sure how long the promotion would last but she would guess through the 15th. I wasn't currently looking to buy one but thought it was a good deal worth sharing. YMMV as it was an AT&T mall kiosk and I know that they may run somewhat independent of AT&T as a whole. I also couldn't find anything about it on AT&T's website, Google or Slickdeals. Hopefully it wasn't a store specific offer, I'll try and ask for the scope next time I'm there.