The top offer on this page is the 50% off selected products code at Shiko. It is best for you when the items you want are in that selected range, because a straight 50% cut on eligible items usually beats percentage deals that are smaller. If the product you want is not on the selected list, the 15% sitewide code is the simpler fallback for mixed carts.
Very small orders under about $12. Use the $5 off every order code when it applies to the items in your cart. On a $12 order, for example, the $5 off code takes $5 from the price, leaving $7, while 15% off would save only $1.80. So the $5 fixed discount wins on low spend carts.
Single selected product where the 50% code applies. If the item you want is part of the selected products for the 50% code, use the 50% code. On an $80 selected product, for example, the 50% code is 50% off, reducing the price to $40 and saving $40; the 15% sitewide code would save $12 on the same item, so the 50% code is the better choice.
Multiple items where most are eligible for 50% selected discount. Apply the 50% code and it will halve the price of each eligible item in the order, then you pay full price for ineligible items. If only some of your cart is eligible, compare totals at checkout to see if the combined saving beats the 15% sitewide code.
Mixed cart with few or no selected products. Use the 15% off sitewide code or the duplicate 15% order code, since those apply across the whole order. On a $200 mixed cart with no selected items, 15% off reduces the total to $170, saving $30. That will beat the $5 off code and the 20% selected code if none of those items are in the selected list.
When buying one of the small product deals. Use the $10 off Spaoxy Gel or the $5 off Mild Cleansing Oil deal when you are purchasing that specific product, because those fixed item discounts apply only to the named product and can be larger than a sitewide percentage on that SKU.
Shipping and free gift thresholds. If your order is over $60, you get free shipping; over $120 you qualify for the free gifts offer. Factor those thresholds in when comparing totals, since free shipping or a free gift can be worth more than a small additional discount.
When choosing, always total your basket twice: once with the 50% selected products code, once with the sitewide 15% code and once with the $5 off code if your cart is small. Take whichever leaves the lower total. The codes are redeemable once per order and customer where the terms state that, and no combination guidance is given, so treat each code as a single choice at checkout.