I've always found the easiest thing is to meet them at their car and say what's happening that day. Hi, how are you? Thanks for coming out. I've decided to do shots but I'm just going to wait and see if they foal. Do you have some banamine paste you can leave me in case of a reaction to the shots? I've never had one have a problem with it whether it was me changing my mind or adding something on even when it wasn't 'my' vet and they were here for a boarder. I sometimes get 'the look' but that's it. He's probably used to big horses and will be happy to not have to palpate a mini.