Is National Popular Vote Interstate Compact Constitutional for electing POTUS?

[JRP1994]

A lot of people are talking about what is known as the National Popular Vote Interstate Compact. The NPVIC is a proposed state-driven method of rendering the electoral college obsolete. In a nutshell, this is how it works: when a state adopts the NPVIC, it agrees to award its electoral votes to the winner of the overall national popular vote. Once the total number of states having adopted the NPVIC equals or surpasses 270 Electoral Votes, the Compact would go into effect, thereby awarding 270+EVs  to the winner of the national popular vote. As it currently stands, 9 states + DC (136 electoral votes) have adopted the compact.

My question is this: is such a compact Constitutional?

1) Article 2, Section 1 of the Constitution states that "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress". The pro-argument is that states are given the explicit right to determine how to choose electors, so long as other Constitutional clauses/amendments are not violated, and because the compact does not discriminate/infringe on voting rights, and because each state adopts it individually, it is Constitutional. 

2) Article 1, Section 10 of the Constitution states that "No state shall, without the consent of Congress, lay any duty of tonnage, keep troops, or ships of war in time of peace, enter into any agreement or compact with another state, or with a foreign power, or engage in war, unless actually invaded, or in such imminent danger as will not admit of delay." The counterargument is that the NPVIC is, inherently, an agreement/compact, and unless it is approved by Congress, it is unconstitutional.

So, without rendering a judgement on the merits of the IDEA itself, my question is -- from a Constitutional standpoint -- the NPVIC constitutional?