They very likely don't do FREE refills at all. You probably just got an employee who was feeling kind and just let you have it a time or two for free. I would just accept that and pay the refill price.
Not sure about coffee but i asked for a refill on my tea and they said they don't give free refills on tea. Coffee and tea are cheap to make. Both should have free refills.
I'm not certain. You should always be able to get the refill price on regular coffee if you provide your own cup is my understanding. I had this same problem with macchiatos,reg coffee and teas. I was told they quit doing a refill price on the macchiatos which is understandable on the specialty items but not so much on regular coffee and teas. They market their cups and glasses for advertisement of course but us consumers buy them generally for the convenience of the refill price which is under $2 locally. I've ask the barista/management this question and I was told I was not given refill price on regular items due to a inexperienced employee.
If you dine and you should always get free refills on your coffee it shouldn't be every now and then
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