We are hiring a musical group for our cocktail hour. After talking to several groups, we noted that all but one had a 2-hour minimum. We're fine with this, and picked the best group and will happily pay them for 2 hours even though, due to our timeline, we will only use them for 1-hour as this seems to be the norm. We've also let them know ahead of time we will only need them from 530-630pm, in case they want to book another gig later in the evening.
But we're unsure what the tipping etiquette is. Do we tip them at all, considering we are paying them for an hour we are not using them? Do we tip them 18% of their hourly rate (i.e. tip them for one hour)? Or do we tip them 18% of the entire (2-hour) bill?