I believe they are ok for any kind of merchandise that's legal to mail. There has always been a flat rate domestic Priority envelope. It used to be at a rate of $3.50 for whatever you could fit into it. I received one stuffed with tokens one time. Must have weighed 5 pounds!

Be aware though, that when the new rate schedule went into effect last year, that there is now a one pound Priority rate of $3.50. If your package is less than a pound, you're still a little better off to use the regular Priority envelope or box and save the 45c.