Can't find how to purchase the ticket for Provins

Hey everyone, I'll be visiting Paris for an event so I'm planning a whole tripe for the city and would also like to add Rouen and Provins. I'm debating whether spending a couple of days in Provins (after Rouen) or just doing a day trip during my week-long stay in Paris.

I must say I'm very impressed by the French public transport sites I've been checking out, but for the love of me I cannot figure out how to get to Provins or purchase tickets online.

It just displays the schedule/itinerary but won't let me purchase tickets. If I search Rouen to Provins, it only lets me purchase the Rouen-Paris leg, but Paris-Provins is missing.

The Provins tourism site mentions there's some work on the railway tracks (only weekends?) but the site they link (Transilien) doesn't let me purchase tickets either.

Bonus question: Rouen-Strasbourg (my next stop) tickets are really expensive, but separate tickets for Rouen-Paris and Paris-Strasbourg (different stations) are fine for certain hours. Good idea or bad idea to purchase separate tickets?