The send special offer button seems to have been removed from that page with the latest Air BnB updates. The best suggestion I have for now is to send in feedback requesting it back. Here is the link; https://www.airbnb.ca/help/feedback
And I was told just like you said that I would have to ask the guest to cancel and send an iquiry or I would have to decline. Seems like it's adding more work for both guest and host, not fun!
The send special offer button is great for adding additional services like linens or adding extra guests. It ccame in handy for us, we miss it!