First (from the linked Citibike announcement), the surcharge for electric-assist bikes is 50¢ for Citibike members eligible for / paying the reduced annual membership; and second (from Citibike's "Electric FAQs" https://www.citibikenyc.com/how-it-works/electric-faqs), "If there are only electric bikes remaining at a station, the additional $2/ride to upgrade is on us."