When we do the opposite - send a cheque for US funds for purchases from our Canadian accounts, we just write "US FUNDS" at the end of the line where we write the dollar amount.
I usually write it also on the top and on the memo line, so it doesn't get missed.
Right now the exchange rate is close with the US rate being higher, so I don't think you will short-change them, even if it comes out in US funds and you will only be out pennies, if anything...
Julianne :O)