Limiting Free shipping discount code to specific rate or carrier

Hi, I regularly go to town markets to sell my goods and I want to convert offline customers to online customers. One way I am doing this is by giving them a discount code for free shipping from the the webshop when they pay at my stall.

However, free shiopping discount codes can only be limited by country, and I only want to offer the free shipping through a specific (cheap) carrier delivering to a local package outlet.

Right now, a user wirth the free shipping discount code can choose any of my shipping rates or carriers, including the expensive ones.

Any experience with this problem?

Regards, Christian