With shipping charges varying so much, I can see why it is hard for a company or a seller to accurately figure shipping. Additionally if you have to find a box, packing materials, or a bubble envelope, there is a cost over actual shipping and I don't resent that a bit.
When I sell items, I have a terrible time figuring out how much to charge for shipping, so I estimate to the best of my ability, allow a little for packaging materials and do the best I can. If you buy from me, I take the package directly to the post office and then send it the fastest way possible while still staying close to the price I charged you for shipping--about 50% of the time I lose a bit of my profit and about 50% of the time I make a little bit over--not much and not intentionally. Often they tell me at the Post Office that first class will get there in almost the same time as priority based on where it is going to. I only charge parcel post for packages that are much more expensive than I anticipated or for those I purposely charged low shipping because they were heavy and would have cost the buyer a lot. A buyer can always request faster shipping if they are willing to pay the additional difference.
My gripe is with companies that will charge $6 or $7 or higher to ship a small packet and then ship it via parcel post causing it to take 2-3 weeks to get to me particularly if I live across the US from them. When the packet is a small bubble envelope and they pay $1.17 to send it on the "slow boat" it tells me their goal is to make money off the shipping. I resent that. I would rather pay a reasonable price for the stamps than to have them present me with a "deal" and then make up the money by shipping it the slowest way possible.
I guess this irks me a bit more at holiday season when time can be of the essence. I don't think it is PMS, LOL!
I want to thank those companies that take this into account and at least are up front or offer options. I think if they were just honest and a person knew what to expect, there would be less of this feeling like they are trying to make a profit off the shipping while making it look like they are offering great sale prices.