[dwcsr]

There are expenses that private sellers don't usually take into consideration.
The bulbs ideally could be shipped for $3 and cost $3 for a total of $6.
Private sellers normally use what ever packaging they have on hand and drop it off for shipping on their way to work. A business has a person that needs to be looked at as a profit center to stay in business. That person needs to make back his pay check plus.
The packaging needs to be paid for. Scales, tape, boxes, dock equipment, all come into play. knowing the weight of all the items you sell take a lot of time to do. None of its free to the business. A private seller doesn't have these expenses. A business makes maybe 20 cents on a $3 bulb order so they do have to make the shipping profitable. Some add a handling charge. So the $3 bulbs with $5 handling and $12 UPS that cost them $6 comes up to $20. They now turn a $0.20 profit into a $10.20 profit that covers the workers paycheck and shipping supplies.

Some very light items that maybe large in size fall into the DIM weight, Dimensional weight, and UPS charges what a package that size "could" hold and UPS charges that rate so a dash pad that is a 3 lbs goes in a box that can hold 25 lbs you get charged that rate for 25 lbs. Gas tanks fall into this category all the time actual weight is 27 lbs but UPS uses DIM weight and charges for 75 lbs.

Percentage of the sale is a very accurate way of estimating ship cost but its a cost over time way to do it. You take sales for a period of time say 90 days and calculate the sales versus shipping cost for those sales and that's your ship rate per dollar sold. It makes shipping on $3 more reasonable than weight plus handling. The problem for the business is that you may not know you lost big money on shipping until 90 days later.