Usually they get for example £1000 to spend on whatever items they choose from the brand website, then on top of that the actual pay/fee they charge (Freya's case I'd guess anything between 10-20k depending on the campaign size, how many posts and which channels etc) and then on top of that the affiliate links. Sometimes deals work in a way that they don't get a beforehand agreed payment/fee but have made a contract agreeing the influencer gets 20% or so of all the sales the campaign has made through their ad. It's easy to track the total sales through their discount codes.
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I personally don't think influencing will ever die off. Platforms and people might change but social media doesn't - it's really up to whether the influencer themselves stay relevant through the years... Freya doing the same stuff year after year is not good for her business.