By bying Fairtrade products you support farmers in developing countries through your daily consuming choices.
Give feedback to the shopkeeper or send e-feedback in the web pages of the shop. Tell that you would be happy to see even more Fairtrade products in your shop.
Offer to tell the shopkeeper more about the Fairtrade system and products and offer to bring Fairtrade posters and brochures to the shop.
Offer to give a Fairtrade tasting in the shop. Ask your local Fairtrade organization about possibilities to organize tastings and for Fairtrade information material: http://www.fairtrade.net/labelling_initiatives.html
See also People and Planet UKs Guide for Fairtrade tastings