How do Chinese exporters handle customs, taxes, and related costs when shipping goods to the U.S., and how much do these add to prices?

Import taxes and surcharges (if any) are the responsibility of the buyer, not the exporter (or seller). In any case the Chinese company would simply sell the item at its sale price and add to this Insurance and Freight.

Upon landing in the US, the Customs will levy a duty/tax on it (if applicable) and add that to the Invoice amount that accompanies the goods and the importer (buyer) would have to pay that.

Under-invoicing is serious offense, so it should be avoided at all costs. Every Chinese seller should be able to provide you with the HS Code (Harmonized System) Code for purpose of duty calculation.