Pathetic Service !

I purchased a Mobicell Hero X tablet from Markhams Phoenix on 14 February 2015 for my wife ( Valentines Gift ). I was charged R2199 for it. On 22 february 2015, the tablet refused to come on. I had a look at my invoice and was startled to discover that I was actually invoiced for a Mobicell Omega X tablet instead of a Hero X. I went onto Mobicell's website and discovered that the Hero X retails for R1799 and the Omega X R2199 respectively. This is an absolute *******. The only reason I noticed this was because the tablet went dead and I inspected the invoice for guarantee purposes ( I did not do so before because I did not want my wife to know how much it cost since it was a gift ). When I called Markhams Phx today 23/02/15, they say there is nothing they can do because the tablet is activated. I'm sorry to say this but that is unacceptable. You guys charged me for a higher spec model and gave me a lower one ( how many other innocent customers are getting hoodwinked like that ? ). Price is not an issue, I have been a loyal Markhams customer for over 15 years ( I have built up a credit of R15 000 with a balance of only R2000 ). NB - Today is the 7th working day.