What's going on with the shipping pricing?

    > Standard delivery (approx 3 weeks)
    > Guaranteed (+$15)
    > Within 25 days (+$20)
    > Within 15 days (+$20)

3 weeks is 21 days, so assuming "guaranteed" just removes the "approx", why would I ever pay more for "within 25"?

Further, why would I ever pay for "within 25" when "within 15" is the same price?

Surely this can be condensed to:

    > Standard delivery (approx 3 weeks)
    > Guaranteed within 3 weeks (+$15)
    > Guaranteed within 15 days (+$20)

without changing the effective pricing; while making the options clearer.