Any other sensors compatable?

@michaeljr1953 Luckily, you have options.  First, a standard entry sensor. Depending on the distance to the base and the material your mailbox is made of, that can work. Note: Metal mailboxes block the signal, as I know from personal experience. If its made of plastic,  you have a shot. Second, on Etsy, there is an existing SimpliSafe customer that has extended the antennae to increase range AND you can slip it through a gap in the door or in a drain hole in metal mailboxes.

Now, on your "wait, they can get wet and it will go beyond the temperature range...... I have a motion sensor in the garage and two entry sensors on my garage doors for years and, in the greater Chicagoland area, work just fine in that unheated garage from 10-15 F in winter and 90s in the summer. 

And to be thorough, please note that you will void your warranty using an entry sensor this way, but for under $15 bucks,...