check this discussion
we are trying to solve a similar problem… it should be straight forward, if a customer is tagged ‘X’, give discount of ‘Y’ at checkout automatically…
it shouldn’t necessitate an app or upgrading to Shopify Plus for $2000 a month…
also check this piece of code out that I found in the above discussion… it at least seems to add automatically a discount code and the customer only needs to hit APPLY… not ideal UI but better than spending a lot of money…
https://wearetmbr.com/shopify-auto-fill-discount-code-on-checkout-page/
hope this helps… and yes Shopify, please add a simple feature to apply a discount to selected customers automatically or allow smaller retailers to create a Wholesale channel without paying a $1920.01 premium…