Yes there is. A asshole mailman might see it and open it if they know there is money inside. Since theres not racking code, theres no proof it even existed. Here's what i suggest keep the money INSIDE the piece of paper. Or use a security enforced envelope which prevents people from seeing whats inside.
Sending it by mail is tedious, send cash NOT money order. If mailings all you can do, it might take a week before getting a donation rank.
Hope i helped, i only lost money once and that was when didn't use a security envelope and left the money outside the paper i was sending. Lost 15$ tho.
Back then there were no cards for runescape so i did pay by cash payments to Jagex.