Be specific about what invoice you paid, the amount of the check and the date on which it was sent. Offer to replace the check if it doesn't arrive within a reasonable length of time.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I received an invoice number <invoice number> for <$amount of invoice> on <date received>. This letter is to advise you that I sent a check number <check number> for <$amount of check> on <date sent>.
If you do not receive the check by <date> please call me at <telephone number> and I will stop payment on it and promptly reissue another. If I do not hear from you by then, I will assume that the check arrived. Thank you.
encl: <List of enclosed items goes here>