My Spider Sense tell me this is not really about locking in your rate for ninety days, because if it was, they’d just sign the contracts NOW, wouldn’t they? It’s not like the big event they are planning is in danger of being cancelled. They’ve locked-in the hotel and the transportation already, I’m sure. The publicity has gone out, or is about to. The ads are bought and paid for. This is happening.
I think this is a probe to see how firm you are, so they can try to negotiate your price down. Either they already know you are the guy for this, and are (redacted)-ing-around, or they have one of your competitors on the top of their list and you are just the “official” (required by accounting rules) benchmark comparison price source, not the real candidate for the job.
My take is, re-draft a new bid with the 90-day guarantee and raise the price eight to ten percent, to reflect the extra risk. You are neither a bank nor an insurance company. They sense you really want this gig; that’s risky for you. You can’t bee too eager. Rule two of business negotiation is; they have to believe you can walk away from a bad deal – if they don’t believe you could walk away from a bad deal, they own you. And that reputation will poison your future deals in that market because now people know you can be rolled.