I've never sent a letter, but your staff partner should be able to get you stickers that say "I dressed in support of Relay For Life".  I work for an accounting firm that does not have regular jeans days and we hand out a sticker to each person who pays $5 to wear jeans on select Fridays so we know who has paid.

I work at a bank and we do casual week every year. The way we do it is we made really cheap buttons that say "Casual for Cancer" and have the Relay logo on them. You pay $5 for the button to be able to wear it and jeans for a week. We made over $800 on this fundraiser last year and we are a small community bank. I don't have a template for a letter but I would recommend putting info about how important Relay has been for your business and how successful this fundraiser has been for others. Good luck!