We're supposed to reward good behavior and punish bad behavior, but I was wondering if the threat of punishment was enough?

For example this girl was supposed to come over to my place tonight. We made plans on Wednesday but she texted me yesterday saying that she needed to cancel. Asked to reschedule for Saturday or Sunday. I texted her back saying it was going to cost her (without saying what exactly), and Sunday night.

She seemed pretty enthusiastic after that which makes me wonder if the threat alone is enough. Or do I need to follow through all the way? Thanks