The code P0446 is, “Evaporative emissions system vent control circuit [has a problem]”. It doesn’t say to change anything. Last summer, parts were changed. It’s unlikely that those new parts fixed anything. What is more likely is that they did something else which “fixed” the problem for a while. It sounds like this latest place has only scanned the code and jumped to the plan to change some expensive parts, again.
On this site: http://www.toyotanation.com/forum/t74076.html scroll down and read Qslim’s response. Ignore the others. Check all the vacuum hoses on, and about, to, and from, the charcoal canister, for cracks, splits, and deterioration, and connection.