When i switched from trad to wfp, i would tell the customer that the windiws would virtually stay clean regardless if it rains the day they are cleaned.
For new customers, I had a welcome pack and within that was an a5 leaflet which in part said about working in the rain and referred them to my website for further info if they so wished.
(It also mentioned another controversy, locked gates being open via a text night before and if not, 75% will be charged even if just front windows get cleaned 😏)