I would be very leery of sending money via Western Union - it's the money transfer method of choice among scammers. I don't know what the Ukraine transaction consists of so I am not commenting on that.
But when someone asks you to send money by Western Union you can kiss it goodbye, along with whatever you were hoping to buy. Like throwing cash off a tall building.
I have been using Paypal for over 10 years and have never had a problem.