You can definitely get a refund for that. Realistically, DD will just refund the charges and then ban your account. Cheaper than fighting over a $15 tip, especially when disputes cost them whether they win or lose.
While you can almost always get a refund for any small charge if you dispute it, it doesn't mean it's technically allowed by the terms, or as part of mandated laws. Laws around this generally only say the credit card company has to investigate your claims, but what they do, if anything, or how hard they "investigate" is not enforced. In practice instead of actually investigating a small $15 charge they'll just refund you.
Given a large enough charge, they will definitely not refund you unless it's like a 100% indisputable billing error case (like you send an invoice you received saying $100 but the statement charge shows something else).
That being said, technically it's not allowed as you authorized the charges and maybe didn't get 100% of what you were advertised, but I'm not sure you can get a refund for that.