$20.50 * 300 = $6150 subtotal

$6505 - $6150 = $355 freight

$355 for express (most likely DHL, FedEx, SF Express, etc) isn't too bad. If they are quoting EXW it just means that they aren't taking it to the port, etc. In this case, the express shipper is picking it up (they probably have an established daily pickup) and that their liability/risk ends when the freight carrier picks up the item.

In my experience, a lack of proficiency in English is extremely common with Chinese manufacturers and you'll need to clarify exactly what they mean during the entire purchasing process. If it were me, I'd ask them to send a detailed (line item) quote for the products and then ask for them to list the cheapest door-to-door air freight cost on the quote, along with the total number of cartons, the dimensions, and the weight. Once you have the dimensions you can contact a freight forwarder (or in the case an express/air shipper) and get your own quote.

I'm not sure if you're aware but 10/01 to 10/07 is a major holiday in China and this may delay your shipment and communications with them.