To cite sampleSelection in publications use:

Toomet O, Henningsen A (2008). “Sample Selection Models in R: Package sampleSelection.” Journal of Statistical Software, 27(7). https://www.jstatsoft.org/v27/i07/.

Corresponding BibTeX entry:

  @Article{,
    title = {Sample Selection Models in {R}: Package
      {sampleSelection}},
    author = {Ott Toomet and Arne Henningsen},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {27},
    number = {7},
    url = {https://www.jstatsoft.org/v27/i07/},
  }