Don't send cash payments through the mail, but if you can't avoid it, be prepared for the risk you are taking. Sometimes, letters and packages are lost in the mail, and if there's cash involved, you can't do much.
If you should put cash in the mail, many factors cause the envelope to go missing, making you to lose the money. Factors include sending it to the wrong address, a dishonest person denying receipt of payment, or it getting lost in the mail shuffle.
One way to ensure full reimbursement for the cash sent through the mail is to have it insured. By insuring the value of the amount you are sending, you will be able to track the package and submit a claim with the post office for the amount you insured.
If you do not have an alternative method to send a payment, such as paying with a credit card, debit card or a check, you can check with the post office about purchasing a money order. There will be a small fee for the purchase, but you will be able to track the item, have proof it was received and be able to get your money back should the money order go missing.