How do I write up a marketing/consultation fee into a large deal?

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I have a good problem. I negotiated a deal with A class multi-family units in Florida by connecting a buyer and seller and their brokers. This deal is now in the pre-due diligence phase with letter of intent being written by the buyer side. He wants me to send an email stating my “marketing fee”. How should I word this email? Does it have to be in a contract format? An investor sort of mentor of mine said to ask for 3-5%. There are about 8 A class multifamily properties for sale - all being sold for millions (60,000,000). I am excited as letters of intent are being prepared etc.. I deserve to be compensated but I’ve never completed a deal like this before. I don’t want to sound inexperienced or get taken advantage of. Please advise.