They will have to come up with another rent payment. "lost in the mail" is not the landlord's problem.
Let me point out to you a very favorite tenant trick. It is called, "The check is in the mail". It's a very common trick right around the holiday season.
Even good tenants might try this after they decide to move out. The plan is to stall you off for the last month before they skip out, so that they don't have to pay the last month's rent. Expect to find a lot of damage when they do this, because they already know they won't get their deposit back.
I like rent paid in personal checks, because if the rent gets lost in the mail, the tenant just has to stop payment and write me another check.
If the rent goes to a post office box, all the tenant has to do is to mail the rent at the post office and it doesn't have any opportunity to go astray. They are already out and about in order to purchase the MO.
Even if they show you the receipt for the money order, it means nothing. All it means is they purchased one and sent it to someone. It doesn't mean they sent it to you.