Park West Gallery review: Overcharged shipping on multiple shippments

I've purchased artwork at various cruise ship auctions - prices are advertised as bid price, which includes the artwork + shipping. I later realized that the shipping cost was incorrect (exactly double) on 2 invoices. First response from PW was that the costs were correct - even though I was charged more than the bid price. Then I was told that the charges were because I had changed the frame on 1 piece, no answer on the other.
No dice - first, there is a specific place on the invoice for custom frame charges, if they were applicable.
2nd the shipping cost is exactly double the cost charged to ship other similar pieces - not a coincidence.
3rd, companion pieces with the same framing did not incur any additional charges - and I was told at the time of the sale that there would not be any additional charge.
I've had no further response from PW