It's pretty clear - there's an entire paragraph devoted to it on the
About page (in the top section of content, after the banner), tooltips on the search results, a price breakdown, an FAQ question, and the fee is baked into all the prices from the first opportunity (no drip pricing):
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If I understand
@thomasheywood, they're not so concerned with the fee being hidden, but the way it's often described as 'no booking fees', which seems a bit misleading compared to 'only a booking fee when we save you money'.
E.g.:

I find it more of an issue on rail forums etc where for a long time Trainline has been slated for charging a (capped at £3.99) booking fee and in the same breath promoting sites like Trainsplit. Both sites sometimes charge no fee, and sometimes charge a fee, and both sites cap their fee.
(The main argument against Trainline imo is actually that their split ticket algorithm is poorer, but don't want to turn this into a Trainline post per se)
Sometimes Trainsplit sites save me more money than competitors; other time competitors win because they find the same splits with lower booking fees.