Advertised sale price not correct at till - Consumer Complaint for Clicks Store - Head Office

Gaviscon products are currently advertised online an in-store as 20% less, but at till point only 10% is deducted, even though on till screen and receipt iy shows minus 20%, where in fact, if you do the math, only 10% is deducted. This is not accurate advertising, please correct on your system.