Availability Calendar Checkout Integration

I have browsed a number of forums touching in and around this issue, but I am still looking for further and more specific assistance. 

https://harp-chicory-kphm.squarespace.com/
password = password

GOAL: When a user adds an item to their cart, I want to limit the times when we have to go back to the client and say that we're unavailable. So, when they are ready to check out, I would like to do 1 of 2 options:

1. Add a Button next to "CHECKOUT" that allows the user to first ensure that we have availability on the day they wish to pick up their board. Once they have reserved the time/date, they can then go back to the Checkout page and continue with the CHECKOUT button (note: I already have my Acuity set up to redirect back to Checkout, that works beautifully thanks to @codeandtonic)

2. If that's not possible, I found another thread talking about embedding the acuity calendar on the Checkout page itself. Has anyone posted a step by step for this with clear instructions (i.e. [insert URL here] etc)?

I had added code from @creedon successfully, but realized that once I successfully redirected them to the acuity calendar, they selected an available date and time, it then redirected them back to the cart as I had hoped, but unfortunately they could never actually add that item and check out from there. 

Thanks in advance!