I would have just said
Hi
Your first clean price would be £xx
Then future cleans would be £xx every 4 weeks
Im next in the area in two weeks, if you wish to go ahead, let me know and i will text you the evening before to confirm when im coming.
Thanks
The rest of it, working in the rain, dumping them etc they will find out in due course. i wouldnt worry about it, charge double for the first clean and take your chances.