A “compact agreement among the states” is called a Constitutional amendment.
The National Popular Vote Interstate Compact is not a Constitutional amendment.
Article I-Section 10, Clause 3 of the U.S. Constitution specifically permits states to enter interstate compacts. There are hundreds of major compacts currently in force (and thousands of minor ones), as can be seen at
http://tinyurl.com/3ra7elc
http://www.csg.org/programs/ncic/default.aspx