When shopping in opportunity shops (or charity shops), pay what the item is worth to you, even if that's more than the asking price.
Op shops are a great, affordable way to develop your own unique style. For this reason, a lot of people shop at op shops who don't necessarily need to. There's nothing wrong with that, but remember that op shop prices are aimed at people who can only afford a token payment, and any meagre profits they make, go to support the charity which usually involves people who can't afford to pay anything for clothes.
So, if you find a nice shirt, a cool jacket or a funky tie at an op shop, give them a little bit extra if you can afford to. I don't mean paying what it would cost new, but if the tag says $4, and you would have been willing and able to pay up to ten, then be cool and give them the ten. It's worth it just for the expression of surprise and gratitude you'll get from the nice volunteer working there.