I have NEVER had a 'brokerage' fee from china purchases, so this is NOT nearly as prevalent as you seem to make it out to be.
I have no idea how prevalent it is in the US, but it is a common "fee" for international carriage when crossing borders and customs has to be involved. It is extremely common for items imported to Canada, where UPS/FedEx/whoever will slap a $25 (and up, depending on declared value) fee on any package worth more than $25. There is no avoiding it here unless you broker yourself, which means a trip to the airport and a bunch of paperwork and hassle, or using the postal service, or simply shipping to a receiver in the US and picking it up yourself.
I am led to believe this is an even bigger problem in countries with stricter customs rules.
So yeah, it might be uncommon in the US, but you guys are the exception, not the rule.
I do blame the seller since he picked UPS entirely on his own. I'm not sure why you are defending him (are you him??) and, again, I'm not being charged CUSTOMS fees. those are things he has no control over and I understand that. but his choice of UPS clearly puts the buyer at higher risk of being charged the bilking fees from UPS.
There are basically two options here for why you got charged brokerage, and neither has much to do with UPS being chosen per se:
a) This package needed to go through customs for some reason. I don't know US customs rules, but UPS/FedEx/etc. do, and if this package had to be cleared through customs, you will be charged brokerage. Period. There is no way around it with UPS & FedEx. If they can, they will charge you this fee, but obviously they can't charge you for it if they don't actually need to send the package through customs. This will happen with any carrier, it's a major revenue stream for them.
b) Somebody screwed up and
thought this packaged needed to be cleared by customs, and sent it there, but in fact was incorrect and it should have been sent through without a customs visit based on the US rules. If that's the case, it was human error, you should contact UPS and clarify, and if they can't show why the package needed to be cleared, then you should demand (and receive) your package without fees. Human error could happen with any carrier.
So, I don't think the seller is really at fault for this. Either in the case of a), you weren't aware of your own country's customs rules, or b), someone at UPS made an error and you shouldn't really be blaming the seller for that.
Edit: UPS's policy for import into the US seems to include brokerage from everywhere except Canada (WTF??), but they do have various fees for special processing etc. which it sounds like you might have hit. What exactly is the line item called out as on the invoice they gave you? I guess the rubidium probably trigger some special inspection or something stupid.