UPS Shipping Rates: Higher Late in the Day?

Is it my imagination, or do UPS shipping rates increase if you purchase shipping labels later in the day?

It feels like UPS shipping labels are less expensive in the morning, and they get more expensive later in the afternoon. Maybe there are other factors at play, though, such as customer location, etc.

Our packages are very consistent in size and weight. They're not large or heavy; they tend to be slightly smaller than a shoebox and less than one pound.

Thanks!

-- Tony