To take advantage of this discount, you must choose the pack of 100 prints when you select your product (and not individual prints). If the price is higher than expected, you haven't chosen the pack of 100 prints.
And, of course, you must order a minimum of 100 prints.
You need to empty your cart and start your print order again. You can choose the pack of 100 prints when choosing your product. Please find a screenshot attached.