Next thing will be “I can get a magnaclean off a bloke on EBay for £80 instead of the £120 that you’re quoting” then “Just how many screws have you quoted me for, it’s not a full box of 200 is it?”
Just shows how naive some people are. They think any money above the cost of the main item to install is all labour.
Your reply was good and nice and polite.
It would be tempting to tell him to buy the boiler from the internet (to save himself the hundred plus quid) and you will install it.
Then when he comes back to you in a couple of weeks, just tell him other customers have now taken his time slot.
Personally, if I received a message like that, I would have replied:
Thanks for bringing that to my attention.
I rescind my quotation.
I have made a mistake and my quotation price is now £300.00 more.
( gee I had trouble finding that £ symbol - we went to $ in the mid sixties)
Please sign the new quotation and deposit 20% deposit into my bank account and we will proceed with the installation
He shouldn’t forget that you will need to power flush the system too. Some of the people don’t understand what’s actually involved to do a job good and neat. These people see only the money they pay nothing else. What I would do next time is, send him a quote broken down this will avoid confusion.