I just placed my biggest (in terms of dollars spent) order at JLC, and even though the tariffs aren't 100%, the net effect is that my order cost was literally doubled due to an avalanche of extra costs.
It looks like the cheap shipping option, the "Global Standard Direct Line" is only available when the merchandise total is $59 or less. And the merchandise cost used is pre-coupon. So it's easy to stay under $59 for things like bare, HASL-finished, two-sided PCBs. But it's really hard to stay under that amount if doing PCB Assembly. The PCBA service alone has a $25 flat fee, and ENIG finish adds another $16 or so. Throw in a few extended parts with the $3 feeder fee per, and it's practically impossible to stay under $59 for PCBA.
Going over $59 requires shipping by one of the major carriers (DHL, UPS, FedEx), which is $30-50 even for a small order. Then the tariffs, plus additional carrier fees for their added overhead. Plus sales tax. In my case, all these extras literally doubled my cost (actually slightly more than doubled).
If I was doing this as a business, I don't know how I couldn't pass the costs on to customers, unless I had enormous margins.