Mightly is currently showing the up to 60% off plus extra 10% off voucher at the top of this page, which is best if you can apply it to items already reduced in the sale and want the biggest possible percentage saving on a larger cart. That code is aimed at shoppers buying selected sale styles where the combined discounts apply. If you have a small order or need a guaranteed flat saving, one of the sitewide codes could be better.
For very small carts under $50. If your cart totals around $40, the $10 off sitewide code will usually save more than 20% off. For example, on a $40 order, the $10 off code reduces the total to $30, so you save $10. The 20% off code would reduce $40 to $32, saving $8, so pick the $10 off code.
For mid sized carts between $50 and $150. Use the 20% off sitewide code for most items not in the sale. On a $120 order, 20% off takes $24 off and leaves $96, while the $10 off code leaves $110, so the 20% code is better for a $120 cart. If you are a new customer you can instead use the 15% first order offer from the site or sign up to the newsletter for 15% off first order; both give 15% off only on your first order so compare 15% versus any sitewide code when you are new.
For orders of sale styles. The sale prices showing up to 75% off are the place to start. If the item is a sale style, the site sale price already applies; the up to 60% off plus extra 10% off voucher at the top of this page is the one to try first because it explicitly mentions extra 10% off your order on selected products, and that is the only code that targets sale stock for additional reduction. If an item is in the final sale listed as up to 50% off, that final sale label applies to those styles, and might be separate from the up to 75% sale line.
For Rebel Girls collection shoppers. Use the 20% off Rebel Girls collection code if those items are in your cart, since that code is specific to that collection.
For shipping concerns. If your order reaches $100, you get free shipping, so a slightly larger cart that reaches $100 and uses the best percentage code can be cheaper than a smaller cart that only uses $10 off.
When choosing between the applicable offers, calculate the final total: start from the original price, apply percentage discounts in order where they are allowed, or subtract a fixed $10, and take whichever yields the lower total. The newsletter and first order 15% offers apply only to first orders, so treat them as alternatives rather than stackable unless the site terms say otherwise.