“…the average person treats a price ending in .99 as if it were 15 to 20 cents lower.”
The tendency is called left-digit bias, when the leftmost digit of a number disproportionately influences decision-making. In this case, even though the real difference is only a penny, research shows that, to the average person, $4.99 seems 15 to 20 cents cheaper than $5.00 – which results in selling 3 to 5 percent more units than at a price of $5.00"
Why Literally (Almost) Every Price Ends in 99 Cents
https://en.wikipedia.org/wiki/Psychological_pricing
EDIT: The left-digit bias is not just pennies / cents. It applies when going from $99 to $100…$399 to $400…$999 to $1000 etc.
EDIT 2: If you have a car for sale and you want $10,000 for it are you listing it for $10,000 or $9995?
If it was say fruit, or beer in a bar, one isn’t counted that close bulk and the other, you could just wait till you sell six and then ring it through as a six pack to go.