for once a (relatively) novel question about tipping. usually we hear the same question (how much and why, etc).   of course, the whole point of tipping is to do it afterwards as a sign of appreciation for the service you received (although in some service industries it's not uncommon to tip upfront to insure good service -- example on board a cruise ship!)

i would hand her the tip very casually (concealed in the palm of your hand with a handshake/kiss) with a comment like  "this is for the flowers i forgot to bring you" thus making it clear that the "donation" that just exchanged hands was NOT for sex (which arguably it wasn't, it was for her being "nice to you")

the reason you can be more relaxed tipping AFTER the fact is that by that point it's already late for (most) LE agents to even attempt an arrest (they already partook of an illegal activity themselves) but bear in mind i'm not a legal expert, but it's fair to say i'm not too off the mark on this point

If you've made it to the end of a date i wouldn't be concerned with the LE aspect of it.
Usually i'll remove my watch, bracelet, something on that order and set it on a night stand or dresser. Then, if i'd like to leave a tip, after i've dressed, i can discretely place it where my watch was when i pick it up to put it back on.