I can't answer this for sure but I'm guessing that yes, the exact amount is needed. A bank or grocer should be able to give you change, although I'm not sure why you wouldn't just get a money order if you were going to do that...

 If you overpay they'll stick it in your account as a future credit... at least Im pretty sure thats how it works.

 That makes sense. I personally don't like the idea of sending cash and risking it getting "lost" somewhere...so I myself would avoid sending more than was necessary.

 Theres also the option of buying a gift card affiliated with a company like visa, and then buying a gift certificate from the site... Ive done that with gift cards in the past. 

You can just send bills and they put the change in your account to use on future purchases. I sent concealed cash plenty of times when I didn't have a card to use and it always got there.

 I'd be sure to use a security envelope. Its the ones that have usually a gray or other color inside so that folks cannot see through to the contents.  And fold the currency in a sheet of paper.  Also.. performing good juju at the mailbox might not hurt either.