I'm pretty much in the same boat as Busfact. We do, however, send out invoices with a paid stamp quite often, mainly because with "unknown" clients we don't release final documentation until we've been paid - so adding a "paid" invoice to the bundle we're sending anyway is no big deal.

There are a selection of customer messages loaded and we'll include the one most appropriate at the time of sending - which includes a "thank you for your business" message

If there is one area where we do go against the grain somewhat, it's with our statements. The statement run is done 14 days (or first working day thereafter) before the end of month and list open transactions (unpaid invoices and unallocated payments) rather than a period transaction list. I've found it improves timeous payments of accounts tremendously.

The only "hitch" is every now and then someone will request an end-of-month statement, mainly to do with audit requirements on their side, but the extra work is no burden at all really. For those ones we send an open balance and a period transaction list type statement.

EDIT: And invoices are dated on the day we completed service, except for standing order clients which are generated on the first day of the month.