Bullshit. You can't "easily find it out from DHL". Even contacting DHL can be colossally difficult (speaking of experience).
To prove otherwise, please provide a reference. And why should you, when you are not their customer?
http://www.lmgtfy.com/?q=does+dhl+express+include+brokerageIn the Google preview for the first result at my location, without even clicking the link, it says: "At the time of delivery, the recipient is provided with a brokerage invoice detailing the duties and taxes that DHL Express has paid on their behalf, plus a brokerage and transaction fee."
Couriers like DHL are notorious of the fact they make different kind of agreements with different customers, different terms&conditions, different pricing... And everything is volatile. It's a can of worms for someone not used to dealing with them.
If you thought this, why didn't you sort it out yourself before ordering? Also, it's not really true. In the vast majority of cases, one of the standard products will be used, and it will either not include brokerage fees (e.g. DHL Express) or include them (e.g. FedEx International Priority). When this isn't the case, it's almost always because the store has made a DDP arrangement of some sort, such that the duties are prepaid on your invoice with the seller, rather than by the courier, which would be clearly indicated. I've seen sellers just say 'FedEx' rather than the exact service, or actually ship using a different service than was requested, both of which would be legitimate complaints, but neither is the case here.
DHL provides both types of services, brokerage paid by the sender, or not paid by the sender. The only one who can inform the customer about the exact contract with DHL, is the only party who has made the contract with DHL: JLCPCB.
"DHL International Express" means something, and contains the information you need. There is no special arrangement between DHL and JLCPCB, they are using a standard service that you can find the details on at the website. And anyway, the safe assumption is that customs clearance is not included, in lieu of other information. You should assume it is not included unless you know that the service includes it, or is DDP (where you will see your local taxes on the invoice). In this case you didn't know it was included and could have asked JLC or DHL.
The chain of contract is trivially simple: JLCPCB buys a service from DHL, and the customer buys a service from JLCPCB.
Exactly. You've selected a DHL service, and JLC is just reselling it to you. You will get that DHL service at DHL's terms. It makes zero sense for JLCPCB to duplicate all of DHL's (changing and region specific, so they'd be inaccurate anyway) policies on their own website. You selected the service you wanted - DHL International Express - and that's exactly what was delivered.
JLCPCB should simply inform the customer that DHL may force-sell services to the customer. It's that easy. But, I understand this is hard to see for them because of the cultural differences, and the fact people are reacting to this in so many random ways.
I agree that because customers are often unfamiliar with customs clearance, and the exorbitant fees that couriers sometimes charge for it, that vendors should in general warn about brokerage fees and seek to help their customers minimize them. But this is just a convenience and a niceness for the customer, it's by no means their obligation to do so. Caveat emptor, as it were.