Truworths - Hey Betty Bag - Bought a bag but not satisfied

I bought a Hey Betty Bag of an amount of R440.That bag has a tag "Tru Royalty GET A FREE R150 Fashion Voucher when you purchase one of these bags" but when you get to the tills is not like that when they gave me the voucher, the voucher says I have to purchase again with R450 then thats when I can get the voucher and there is a date where that voucher expires.What that mean to us customers.why is that tags have limited information and what is the word free means..can I get a respond and a way forward regarding these.I also asked in the shop and they said thats the way it is they do not know why there is limited information sooo can I get answers.