eBay won't let a seller open a dispute until day 4 and then you have another 4 days after that to pay.
Most sellers realize that not everyone can be sitting by the computer when an auction ends.
Often auctions end when the buyer is out at work or in the middle of the night.
So long as you pay within 4 days you are fine.
So long as you pay within 24 hours most sellers will be happy.
Read the official policy here.
http://pages.ebay.ca/help/policies/unpaid-item.html