@Emily128 That's just confusing, and makes zero sense.
@James261 The reason why the rate jumped is because your discounted monthly rental price is no longer valid once this type of alteration is made for nights booked beyond one calendar month. It has either reverted to your nightly rate or your weekly rate (assuming you have a weekly rebate set up for your listing, which I doubt.)
So you need to submit a price alteration to the guest, independent of the Airbnb cost calculation, using a pro-rata methodology (as per @Maria-Lurdes0 advised, allowing for your additional fees, etc.) I'd suggest a formula/percentage utilising a weekly discount to establish the pro-rata calculation for the amended number of nights requested.
Or not.
Up to you.
Confused yet? 😉
Message me if you want clarification. 🙂