In the UK this is all spelled out by law. In the US it's not.
In your case, I would insist that they make arrangements for the shoes to be picked up. It is their fault, it's not a matter of you not liking them or anything, it is a definite error on their part.
Have them send a "call tag" where the delivery driver picks them up, or have them send you a collect return authroization.
If they point to their TOS, tell them you don't care, they made the mistake and you expect them to make it good. If not, do a chargeback, or file with PayPal or ebay or whoever.
Other than that, there are many sellers who do state that the buyer is responsible for return shipping. I naver have, but many do. It's one of the things to look at when making a purchase. Theo old Let the Buyer Beware comes into play.
Not xo in the UK, they have a law directly related to mail order type sales that makes it the seller's responsibility.
Just stick to your guns, you seem like you know how to do that fairly well. Treat them like you do some of us on the forum.

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