So you are saying I should purchase the combo ticket for June 11 with
no reservations, pay, and then go back and make a reservation the
climb for June 13? Wanted to make sure that was a possibility before I
wasted 30 euro. The booking site always wants to default me to buy a
combo ticket and make a reservation all at the same time.
Yes, I'm saying purchase your online ticket FIRST for June 11. Your online ticket will have a code, and I believe you can use it to book your climb for June 13.
https://en.grandemuseodelduomo.waf.it/museo_dett.php?idtour=8484
See the text on the side which says "Have you already purchased the Cumulative Ticket?
Book your entrance to the monuments!" I'm guessing this link is for people who forgot to make their reservation when they bought tickets or, like you, want those for a different day. They make it possible to get to that page from here as well, through the "booking now" text below "Do you have already bought the ticket?" paragraph.
https://www.museumflorence.com/plan-my-visit
So after you've purchased tickets, click on that and you'll get to this page.
https://en.grandemuseodelduomo.waf.it/booking.php
Click on "See Dome Schedule" and click "change the day": a calendar will pop up. Choose June 13. You can see that there are still openings for many time slots. Now, I can't test this because I don't have a ticket code but try entering your code into the box which says "Insert the number of the ticket" (the graphic below it shows you where to find the code on your ticket).
Hit "proceed". The next page should allow you to choose an open time slot. Give it a try? I'm sure you can back out if it doesn't work. If it does work, you may have to do this for each ticket you've purchased. Let me know how it goes?
It does mean spending the 30 euro up front. If you really don't want to take the risk, then email them for help:
[email protected]
To add: you will print out your tickets (and I'm guessing your reservations as well) and bring them with you. If there's any way at all to make a little time for the baptistry, please do so. THAT one is a real piece of work.