The elves and the shoemaker - Vera Southgate

Once there was a shoemaker and his wife; they were so poor he wondered what would become of them.
One night before he went to bed the shoemaker left out his last piece of leather and,
the next morning, a beautiful pair of shoes were in its place.
A rich lady passed by, saw the shoes and was so delighted, she paid twice the usual price.
This continued day after day until the Shoemaker and his wife resolved to discover the identity of the mysterious cobblers. Who were they?
How did they sew so beautifully?
And how could the Shoemaker and his wife ever repay them?