Pre-tip them as planned. As a 'worker dude', the cost of everything being so high makes people think i get a fair slice of the pie. We do not get 1/10 of what would be considered our fair share of our slice. 
So unless they know you from last time because you stood out as a great customer one way or the other, tip upfront. The whole concept of tips is backwards if you think about it. The waiter/worker goes above and beyond with no clue if you will recognize it ? Inform them you are buying lunch / happy hour, depending on when they show up.
Then watch them a second time. Next year will be your 3rd rodeo and we will only have to fill in small blanks.
So unless they know you from last time because you stood out as a great customer one way or the other, tip upfront. The whole concept of tips is backwards if you think about it. The waiter/worker goes above and beyond with no clue if you will recognize it ? Inform them you are buying lunch / happy hour, depending on when they show up. Then watch them a second time. Next year will be your 3rd rodeo and we will only have to fill in small blanks.