I take it you have not been in the hosting business for long.
You should have other quicker payment methods available (credit cards, paypal, wire transfer, bank transfer, WU etc). Another thing you could do in ask them to scan a copy of the check and email it to you.
You can cap your client's connection and that will help minimize your losses if they don't pay and not make them as angry (if they did send the money) as they would be if you just turned off their web site/server(s).
You can also ask them to use less bandwidth and explain the situation and your concerns to them, if they really sent the money then they will try to work it out.
There are many more things that can be done in situations like this and to prevent situations like this, it all depends on how well you know your client and how confident you are that they really paid.
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