Marked at one price charged more

When to buy groceries for home saw toilet paper on Special for R50 so took a pack. When I went to pay it rang up R89. Went and showed a manager who told me it was for 9 and not 18 pack but all the 18 packs were under that price. I spoke to the store manager and we went to see the price and conviently they were moving the packs away from the price and I was told I couldn't have it at the price it was marked at.