If you are a consumer (Verbraucher) and bought the shoes from a company (Unternehmer) via the internet, this is a distance contract (Fernabsatzvertrag), § 312c BGB. In this case you have a right of withdrawal (Widerrufsrecht) according to §§ 312g, 355 BGB.
So you can withdraw the contract within 14 days after you got the shoes (§§ 356 II n. 1a, 355 II BGB) ...
If you bought online (so you had no chance to see the shoes and try them out before buying them) then you can return them within seven days. That’s very inconvenient for the seller, but it is the price they have to pay for not having a brick and mortar store.
They have to take them back even if there is no fault.
See this UK government site:
It is aimed at consumers. It says that cancellation fees must be fair, and that you (as a consumer) can't expect all your money back.
Now if I order a service today (Jan 26th) to start at March 1st, you'll ...