Its important to also see the "background" on the client I think.

If I have a good client asking to pay the outstanding amount in 3 weeks, i'll allways start by checking their payment history and support tickets.

If the client does this over, and over again... I will gently tell him/her that we need to fix this problem, cos of the time and effort it takes from my side. I will propose solutions for him/her, and await response.

Solutions can be changing payment to yearly, sending out invoice notice earliere 1 month before due date... and so on...

If the client is a "good" client, i'll in most cases accept the delayed payment... Since this only can be considered as good support.