I would hope what was ment was that they waited for 45-60 days to follow up from the original invoice sent showing the 30days.  Right???? Not safe to assume that but if we don't then the next thing to come is sham on you for giving them so long to think about paying you.  Good Luck!!!

The further you let them go the longer it will take to get your money, especially these days with everything getting worse by the day. 

I think a new blurb on the invoices is appropriate - " do to the economy we now require payment due upon receipt" or something like that.