Which when you crunch the numbers works out more or less to the sales
tax they didn't pay when they bought that equipment online. Got it.
Did this tax change unpredictably, or am I still safe saying it's
deceitful to add a fee for it on top of the advertised and contracted
IMO, there's no reason to expect this tax to be directly passed on to an end customer just like there's no reason to expect the company's income tax to be directly passed on to the end customer. It's a cost of doing business and needs to be included in the up-front negotiated rates.
Sales tax is somewhat special in that regard as it occurs right at the point of purchase and is, in some areas at least, REQUIRED to be itemized to the customer.
If it's not in the contract, I'd try to get out of it.