The IV (Immigration Visa) Fee bill is US$380. That is the payment for the visa. Once you pay that, they will send you the Instruction package (packet 3 for K-visas) with the DS-230 part I form and the instructions about Police Certificates, vaccines, etc...
Sometimes when you get the Interview Package (Packet 4), they will tell you to pay US$380 for the visa. You DON'T have to pay for that again. Once you send the IV Fee Bill check to the NVC, you are set! I guess it's just standard procedure to ask for that payment. But if you paid it to the NVC, then you don't have to pay it again at the US Embassy/Consulate.
I hope this can clear things up!
Alix