Business mark up freight....
Pete,
Couple of things may have happened, the low weight would have gone at the UPS minimum, figure a cost of anywhere between $3-6 to the shipper. If your invoice reads shipping and handling charges, it usually means they did not pass on their cost of freight, but marked it up as an additional profit stream.
If it reads freight only, from what I know they legally can not mark it up, but must pass on their cost, of course not all business are aware of this, but some have learned due to past law suits from large customers who regained the differential between the cost of freight vs. the mark-up. That is why most lines for shipping costs read shipping and handling.
One of my criteria for who I buy from is how much they charge for freight, a good indicator of the margins being generated elsewhere.
From the shippers perspective, pulling, packing, shipping, and processing your order of several small and likely low cost boxes would cost the same for a much higher ticket item, so that is why some shipping and handling costs are assessed. Been there did that...
Generally the better customer focused business will have a US Postal option for the small light stuff, and that is usually much cheaper then UPS or FedEx, also indicative of their business mentality towards customers, as this is more work for them.
Doubt if this makes you feel any better, but some food for thought hopefully