I believe the price the Wish was $4 for popcorn, $3 if it was a refill. (it might have been $3 for popcorn, $2 for a refill but I'm pretty sure it was higher). It was only a $1 savings either way; That I do remember.
The person in front of me that had a the popcorn bucket asked if they could get it refilled and the person behind the counter said "sure thing". The person behind me asked her where she got it because she (like me) had hoped to get a refillable bucket. She said she had gotten it at WDW. Then the person behind me asked the Preludes cast member if they could refill any Disney popcorn bucket and he said "Yes. No problem."
I was charged the 18% gratuity, but I was also getting a Margarita in addition to my show popcorn each night, so I didn't really register it. There was only one person behind that counter, serving a loooong line of people for the show. I don't begrudge him the $.72, and added an additional tip on top. Plus he made a mean margarita, so it's all a win for me.
ETA: There were no signs anywhere with refill pricing or rules.