You aren't comparing like with like as e-Bay's £3.38 covers tracked carriage of larger things weighing up to 2kg.
I can't find a price of £2.94 anywhere on Evri's current price-list. A "Postable" item, ie a very small one, costs £2.70 but will go untracked and un-insured. Up to 1kg is £2.89, so slightly cheaper, but the at same 2kg limit that e-Bay apply they charge £4.79.
Besides that Simple Delivery includes protection up to £750 which Evri, Yodel &c charge extra for and the cover has far more holes than that offered by e-Bay. If your item is lost or damaged en route e-Bay refund the Buyer and you keep the money. You can also have your parcels collected by Royal Mail for free — you can't have Evri ones collected at all, which I agree is a problem, but when dealing direct Evri they add around a pound to the cost and, in my experience, fail to turn-up.
Cacas vendit.