when you send invoices by email you can request a 'reciept' so you know they have recieved it, instead of waiting for your cheque for 28days and getting the old "Oh, really, well we haven't recieved it, could you send us another one" routine.
If you use outlook express: to try and make sure they have it, try this:
create your email, When you have finished, click on tools at the top of the screen you have been typing in. ,. a menu will drop down click the part where it says request a read receipt. Then wait, when they open the email it will tell them you have requested a read receipt for that email they may press yes(they dont always do this), the other way to do this is send an image file that tracks the path sent but for a normal person this is way to complicated "Bob hoskins says" "it's good to talk" call them, if you get a big job that mess you about on payment, I would do the following: call them before you go, say we will be cleaning windows (say monday) please can you have a cheque ready for when we arrive, work will comence after the payment via cheque has been made.
Hope this helps a little