Same as above answers. I have thousands of cards printed from solopress.com. carry them in tge back pouch of my belt and simply post just before i clean their door.
I dont have the price on the cards, nor the date, but its a business card size card with my company name and logo and the message
"Windows Cleaned"
So if the customer is used to having one of your cards through the door, simply continue as you did
i do too but i ALWAYS put the date on the slip too....
I personnally dont see the point in dating it. For one reason, they get back home from work or wherever and find the card. Now they know ive been that day! This only doesnt work if they are away, but the same common sense applies. "Oh, the window cleaner has been whilst weve been away"
The second reason, and is trivial, but rather than just pulling a card out n posting, then walk to the next house. You have to mess about writing the date on the card, keeping a pen on you that may or may not work or leak in your pocket etc. To me its just a little thing that i can do without. But like i said thats just my opinion. (GoCardless customers also get an email telling them which week their windows were cleaned. I process them on a friday and put "your windows were cleaned during the week starting mon X")