I don’t think anyone is bashing anyone for throwing in a service or a discount on anything. I’m sure we are all just looking out for your well being. I think it would be better to put a value on the invoice and show them that they got it discounted. This way they understand it’s not free and they just got the extra service done at no cost to them because you see them as a valued customer. I have had a garage throw in a service because I spent enough there. And they too showed the amount on the invoice and showed the discounted amount. To me (I’m not speaking for everyone) I felt valued because they did this haha.
Also on the topic of chandeliers I would have a little flyer printed and hand it to customers with the invoice so they know what you also can do for them when they call you to service their windows again.
Wish you the best of luck!