Diana - I am going to assume that an employee of the RATP knows how fares work and that you likely misunderstood. Allow me to correct the statement for the benefit of anyone reading:
The 30.75€ weekly pass may be purchased Monday (actually Friday of the previous week) through Thursday for the current week. Purchases Friday or later become valid for the following Monday to Sunday period.
The reusable Navigo Easy card costs 2€ and may be loaded with 10 tickets t+ at 17.35€, airport bus tickets, or any day pass. Adding is an option.
When using the RER network, you are limited to stations within Paris (zone 1) when boarding with a ticket t+. The RERs outside Paris and Transilien trains require a point to point ticket, limited to a maximum of 5€ (other than to airports).