Payment can be an awkward time for all involved. Please have your donation prepared ahead of time and hand to the lady or leave on the side table as soon as you arrive.
Not doing this and making the lady ask can be awkward and kind of a downer for the session. Most of the girls will leave after receiving her donation and return soon after. (Only a few minutes). To many times unscrupulous guests have taken the donation when the lady is cleaning up after your encounter and taken with them.