JLC does not have a clear soldermask.
You could simply not have a soldermask also and spray a clear coat on it.
JLC and PCBway are actually very comparable in price for just the basics with the basic colors and everything.
But when you put very advanced setting like dyed FR4 PCB cores and new transparent soldermasks, the cost will double or triple.
The issue isn't that those things are hard to do, it is the number and availability of machines with those settings since switching out a soldermask color is pretty expensive labor-wise and then all following runs through that machine have to use that mask color.