Does adding a fee at checkout sell more?

Yes, it does. Hide a fee until checkout and people are more likely to buy and less likely to shop around, and the research on that is consistent. But if it's a fee customers can't avoid, hiding it is also illegal, and the CMA can fine a business up to 10% of its turnover, or £300,000 if that's higher, and order it to pay customers back.

It's the third part of our series on sale prices and urgency, checked. Each part sets out what the evidence and the regulators say, with a link to each source, and none of it is legal advice.

Why a late fee sells

The evidence review is blunt about it: "Research has consistently shown that when an offer is presented using drip pricing, consumers are more likely to make a purchase and less likely to shop around and compare prices." Shoppers, it explains, "have been shown to focus predominantly on the lower headline price and underestimate the total price of the offer". What nobody has studied is the effect in a small shop.

When a fee has to be in the price

Whenever customers can't avoid it. The first price you show, including in an email or a social media post, should include every charge the customer has to pay to buy the thing. Showing a mandatory charge separately normally isn't enough, and a flat fee on every order belongs in the headline price, even in early advertising.

The CMA's guidance on prices allows one exception. If you genuinely can't work out the whole price in advance, usually because it depends on what the customer wants, you have to give them what they need to work out the total themselves, as prominently as the part you can calculate. Delivery that varies with where someone lives is the obvious case. The CMA also says its examples "are not exhaustive and there may be other ways to comply with the law".

Putting every fee where customers can see it

• List every charge a customer can't avoid. Each one normally belongs in the advertised price, unless it genuinely can't be worked out in advance.
• Check the sale adverts as well as the site. For example, if the advert says £20 and every order carries a £2 fee, the advert should say £22.
• If you show a "was" and a "now" price, put the same fees in both. Whether the comparison itself holds up is the subject of part one, on 'was' prices.

If the same checkout carries a countdown timer, that's a deadline you'll have to keep as well, and later in the series we look at whether the rules reach a business your size. How you price, fees included, belongs in your digital marketing strategy.

If your checkout needs rebuilding to show the full total properly, that's part of our websites work, and how much a website costs sets out the range.