Customer option to upload code without access to the code

I have a product I started to sell using the NANO and would like to give customers the option to upload new code when there may be updates or to make their application more customized. Is there a way for me to email them code to update their product without having to install the Arduino IDE or have access to the code itself?
Thank You!

Yes you could get the compiled .hex file from the Arduino ide and create a command file like bat file in combination with avrdude and sent it like a zip file to the customer and ask him to run the command file which should then automatically flash the chip using avrdude and the given .hex file

Maybe consider using Teamviewer or similar to update firmware remotely.
The customer connects the Nano to his PC via USB and you do the rest.
That way, the customer does not get to see the source code and does not have to operate the Arduino IDE him/herself.

However, if you have a really successful, high value product, then you may have to start exploring other solutions.

You could also base your product on the DIP version of the ATmega328p with a Zero Insertion Force (ZIF) socket and operate a postal update service.