"I received this thermos for free" Seems properly usual to me. "That thermos was for free" sounds undesirable: I'd personally just say "That thermos was free." Agree with Jimi that the most appropriate antonym for "free of charge" is "available." But, "acquired" or "priced" could operate as the alternative of https://docs.google.com/spreadsheets/d/e/2PACX-1vThQYloKZWldp-SOKX4OjBXwRfrgWYaDRrLVem32lfKHmI8mCJiHgR2_0HftTtEvIaeLQv1auoUjdAn/pubhtml