How to sell Custom digital product to one customer only

I’m not sure what exactly you’re selling, but I’ll give my best answer regardless.

If you want them to be able to create it/customize it online from a basic template, this could work, but is more geared towards physical products:
https://wpstorecart.com/app/user-customized-products/

You could also use the built in serial code system in some circumstances, but I’m not sure if it would work for your needs.

We can also custom code solutions for your specific needs:
https://wpstorecart.com/help-support/contact-us/