Skincare is the easiest place to overspend with a code, because a 20% saving makes a second serum feel sensible. My rule is simple: decide what you need before you look at the offers, then use the code on that basket rather than letting the code decide.

Multi-brand retailers are handy for comparing prices across brands, but check the size you're getting, because travel sizes and value sizes can make a per-millilitre comparison look very different. And if you have sensitive skin, a bargain on something new isn't a bargain if you end up returning it opened. Most retailers won't take back used skincare unless it's faulty.