I dont give envelopes or a stamp, if they pay late, they are charged a late payment fee. May sound harsh, you pay your phone bill late you are charged, credit card same banks same.
This is what I have done for a long time, the only ones that dont like it are the late payers and for some strange reason they now send payments on time, I even get sry notes sometrimes with cheques if they are not with me within a week. I class it as a respect type thing, I respect them and they respect me by paying for the work we do ontime.
Ian