Question about Pyrexx Smoke detector PX-1C radio

Good day. I would like to order 4 pieces of this product. According to your homepage, the price is 54 francs per item, which is how it appears in my shopping basket. But on the final invoice, it is suddenly 67.30 Fr. per product. How is this possible? What do I have to do to get the cheaper price? I can send you print screens of the shopping basket and the final invoice. (I also put another non-radio smoke detector in the shopping basket, but that shouldn't be a problem). Kind regards Nicole Devereux