I know why it suffices to show integral ideal (instead of fractional ideal).
Suppose every integral ideal is invertible - (1).
Let M be a nonzero fractional ideal of A.
Then there is nonzero x in A s.t. xM is in A.
Now since xM is integral ideal, there is a A-submodule N of Frac(A) s.t. (xM)N=A by (1). Note that xN is a A-submodule N of Frac(A) and M(xN)=(xM)N=A.
So we can show that every fractional ideal is invertible.
I know why it suffices to show integral ideal (instead of fractional ideal).
Suppose every integral ideal is invertible - (1).
Let M be a nonzero fractional ideal of A. Then there is nonzero x in A s.t. xM is in A. Now since xM is integral ideal, there is a A-submodule N of Frac(A) s.t. (xM)N=A by (1). Note that xN is a A-submodule N of Frac(A) and M(xN)=(xM)N=A.
So we can show that every fractional ideal is invertible.