Pshaw! I routinely mail checks for tens of thousands of dollars, and in the case of taxes (my boss’s, not my own!), hundreds of thousands of dollars. As others have said, you needn’t worry. We old fogies have been doing it for decades. 
That’s kind of what checks are for, you know. How do you think people used to pay bills before you could pay them online? If I didn’t put checks through the mail I’d have to go pay my parking bill in person every month.
The bookstore may sell small opaque kraft/bubble-type containers for mailing CDs and such. But really, as already suggested, just an ordinary envelope and wrapping the check in a sheet or two of paper is more than sufficient.
Count me in the crowd who believes it to be safe. I’ve sent some very large checks via mail, including one for 45 grand. I have never had one stolen.
Just to be safe I wouldn’t put in in a card- there’s a UL that a dishonest mail employee looking for money looks in cards first. 
At work, we send all our checks via registered mail, but we’re talking about amounts up to half a million there.
The insurance carriers we represent send our retainer checks via regular first class mail, usually in windowed envelopes that show “pay to the order of…” and our address. If THOSE get through, I think you’re alright.
Pretty damn safe. I have never had any problems.
It’s probably after the bell, but I thought I’d add this:
Back in 1980-1981 I would periodically mail $100.00 cash to my then-girlfriend (today kaylasmom), along with a three-to-four page Brailled letter, in an unsealed manila envelope marked “FREE MATTER FOR THE BLINID”. Never once did it go astray.
NinjaChick, did you at any time consider wiring the money to the guy?
I am a postal employee. I can’t tell you specifics, but I can tell you a little. In western democracies, theft and mail tampering by postal employees is uncommon. It happens sometimes of course, and those people usually wind up getting caught. Sorters are generally under video surveillance, and while mailmen are not, potentially dishonest ones are faced with the problem that they can only have a small number of articles go missing over a career. In general, even an employee of dubious honesty is smart enough to know that the five dollars in little Johnny’s birthday card from Grandma is not worth loss of a job and a criminal record. If you want to get into petty pilfering, it’s probably better to work in retail. the postal services are a bit too hot.
I can also say that the old stories that we “shine lights through letters” are false. I operate machinery at 40 000 letters per hour. Even if we wanted to pointlessly inspect each letter this way, it would be impossible. We can feel coins in a letter, but not cheques. And most cheques - in this country at least - are marked “Not negotiable” (only named recipient can cash them).
Mail away.
By coincidence, for the first time in my life a check that I mailed (apparently) went astray. It was a $150 check for a water bill that I sent out 3 months ago. The latest bill came, and it showed the $150 as past due. I checked with my bank, and the $150 check was never cashed.
It’s kind of a problem now. I had the following conversation with the water company customer service rep:
Me: If I send you the check for the full amount, will you agree that if you get the old check, you won’t cash it?
Them: Sorry, we can’t agree to that. We just get too many checks.
Me: Because I know what’s gonna happen. If I send you a new check, a month later you’re gonna deposit the old one too and mess up my checking account.
Them: Well, if the check bounces, we charge you $30.
Me: Exactly. And anyway, why should I spend the money to stop payment on that check? As far as I’m concerned, I sent you a payment in a properly stamped envelope on the proper day, and if you don’t have it, it’s not my problem.
Them: Well maybe the postal service lost it.
Me: Maybe, but it’s unlikely, and again, that’s not my problem. I’m going to wait at least 6 months before sending you the money, so that by then the bank will consider the check stale.
(I doubt even waiting 6 months will be safe – the water company probably deposits 10000 checks a day and I doubt they are scrutinized very carefully to see if they are stale.)
The people I see who write checks for smaller purchases and groceries tend to be older people who don’t use debit cards for whatever reason. I’ve also been to financial seminars where the speaker advised us to write checks for every purchase to track spending and keep ourselves honest. I think that’s stupid, so I use my debit card for every purchase. 
Robin
yabob,
As to the question of why women are more likely to write checks for small amounts? The answer is simple–because women carry purses, which thus enable them to easily carry checkbooks. Men don’t carry purses, and generally don’t carry checkbooks, unless they have the intention of using a check.
There are probably other reasons as well–and many women would find the check for $5 or $7 as annoying as you did, but if you routinely carry a purse and a checkbook, writing checks for piddling amounts seems more reasonable than if you don’t.