Agree with Marc, I find out where they are located and figure in a day or two to get the money order (if I don't know them), and then 4-6 days for 1st class.
Then in the same intiial email, i ask them to send me an email when they send payment (that is the request I usually forget).
Now, the last 6 months, when I am buying, well, I am located a ways away from the USA, so I tell them that up front, and if they are good for an extra delay, then we are both good.
I have had more issues with receiving items "late" than payment late, and almost always it is when I have sent paypal or money order?
But again, like Marc said, time goes by, and I send them an email, nice, matter of fact; "was checking on payment?"
Several times they have said, oh i have been busy and haven't sent it yet... I then respond and tell them, thats fine, you can send it via USPS priority (costs $5 bucks) and I'll ship the item or I have another guy waiting and I can send it to him....
I have had it go both ways, they string me along a bit longer or they say, sell to the next guy... but my patience is only so long once I think I am being strung along because they don't have the money or are trying to buy too much
Ed