Hi Oxitran,
I totally agree.
I just want to point out that both Itch and Steam apply the same tax rates to digital products in the UK / Europe, just that Steam include it in the upfront price. You'll see the breakdown in your email receipt. The issue here is the base price is very different on Itch versus Steam.
On Itch = ($7.99) £6.12 + 20% VAT = £7.61
On Steam = £4.82 + 20% VAT = £5.79
On Itch the advertised price is before tax, ($7.99) £6.12, so you end up paying £7.61
On Steam the advertised price is after tax, £5.79, which is the amount you pay.
All I can think is the developer has erroneously set the price much lower on Steam not realising that tax is included in the upfront price, not added at checkout, as is common in the US.