You apply the Provent-a-Mite the day prior so it has time to fully dry out before your boa arrives.
When using Reptile Spray I like to apply it to a white paper towel and wipe the snake with it. There's less waste than if you try to spray the snake, it doesn't stress out the snake, you can get more even coverage and apply it under their chins, and if the snake does have mites they'll stick to the paper towel. I usually wipe them down right as they come out of the shipping box.