They offered you a Buy One Get One Free deal. You accepted it, then asked them to change it to "Buy One Get Something Else" for you. They declined, because this was not the deal they were offering. This is not bad service. This does not mean the staff is uncooperative or not "properly trained". This is the offer, take it or leave it. You may think it's not a big deal for them to give you your own way, but you have no idea of the way the registers are set up or how strict the franchise owner/corporate office may be.

If every business you go to bends the rules for you because you want to be wowed, being wowed in this matter is just going to become routine for you. There won't be any wow potential left anywhere you go, unless your standards become increasingly impossible to meet. So why not just accept this perfectly reasonable service and save the Wow requests for something more out of the ordinary? I think you'd be a lot more impressed by a once in a while, way out of the box move than you would by a free container of sauce.