The Ohwill $20 off voucher at the top of this page takes $20 off your order when you enter the promo at checkout, and it cannot be combined with any other offer. It is best for low to medium size purchases where a fixed $20 saving beats the sitewide percent codes or when you are buying items that are not included in the selected product sales.
Small orders under $120: use the $20 off voucher above. On a $100 order, for example, the $20 off voucher takes $20 off, leaving a final price of $80, so you save $20. The 15% sitewide code would save $15 on that same $100 order, so the $20 voucher is better.
Orders around $120: compare the $20 off voucher and the 15% sitewide code. On a $120 order the $20 voucher takes $20 off, final price $100, saving $20; the 15% code takes 15% off ($18), final price $102, saving $18. The $20 voucher still saves more at this spend.
Larger orders where percent wins: once your cart is big enough, the 15% sitewide code becomes stronger. On a $200 order, the 15% code takes 15% off ($30), final price $170, saving $30, which is $10 more than the $20 voucher. For any order where 15% of the total is greater than $20, use the 15% code instead of the $20 voucher.
If you have sale selected items: the site has product sales up to 27% off and other selected item discounts. If the item is already a selected product on sale, the $20 selected product offers (items 2 and 3) or the sale markdown may give a bigger total saving. For a sale price of $250 reduced by 27% the sale saves $67.50, which beats any $20 or 10% code. Use the sale price on that item rather than the sitewide percent or $20 voucher if the sale applies to what you are buying.
If an offer is limited to selected products or website only: some $20 and $10 offers and several sales are valid only on selected items or website only. If your item is one of those selected products, use that specific $20 off selected product code or the sale price; otherwise use the $20 off voucher or the 15% sitewide code depending on basket size.
Newsletter first time signup: the newsletter signup is a separate option and not the page lead. It can be useful for first time shoppers if it grants a separate first order saving, but it is only appropriate when that signup explicitly gives a discount and you are willing to register.
Choose the option that leaves the lower total. For small to medium carts where 15% of the cart total is less than $20, the $20 off voucher at the top is generally best. For bigger carts where 15% of the total exceeds $20, use the 15% sitewide code. If you are buying a listed sale item or a selected product tied to a specific $20 or $10 code, compare the sale price or the selected product code to whichever general code would otherwise apply and take whichever results in the lower final price.