Shipping rates on site always lower

First time poster, apologies if I'm made any errors here.

I've bought a decent bit, only an occasional seller. One challenge I find on the site is that the calculated shipping rates are so far off from what Fedex/UPS actually charge me. (typically charged almost 50% more, so nearly $300 when the site says $200). There are no additional charges that explain this, as I've had both shippers break down the costs of things like insurance etc. I've experimented with a host of local shipping places in the big city I live in, and honestly YES the different locations do seem to charge me different, and categorize things differently. But all are still way, way beyond this magical number Audiogon comes up with -- like $200 for shipping speakers. Ha!

Am I missing something? I don't need to be told to do a workaround, I need to know what the formula is I'm supposed to use in that workaround. Honestly buyers are very adamant -- if I try to tell them shipping will be more and offer them a flat rate $20 higher to cover even a % of this discrepancy, they act like I'm an 1800s medicine show, trying to pull a fast one on them. I'm just trying to only lose a small amount of money on shipping.

Any insight, beyond the obvious?