Is this job a checking scam? need help!?

So a couple of days ago I applied for a UK company that sells art supplies, mainly beads. They sell them via catalogs to schools and homes. So they asked me for all my information, locations of Fedex or UPS, and cost of stamps. I replied and they said that my job was to ship out their catalogs and mails to customers whom they will give me a list, and that they would provide all the shipping material expenses first. However today, they said that their accounting department had mistakenly shipped me the wrong check of $3,800, instead of $450, and now they want me to cash it from the bank, take $450, and finally wire it back via Western Union. I'm not sure whether they really made a mistake or not since they do have a site, not the best, and he used gmail as his email. I am aware of the checking scam going around, but I'm still not sure. Please help me with any information that you have! thanks.