1 - use the api from your ecommerce app (if not build so you can add recommendation, magent0 has for example extensive api, i think that is possible too)
2 - i dont know what the logic will be in order to create a recommendation from mendix app, but assuming that you have the needed data (or you should import it (via services/sftp/...)) you create and store the recommendations in your mx app
3 - push the data (recommendations) from the mx app towards your ecommerce app by the accepted api service