In my situation very similar to that of the OP, I intend to use the Optum service ($70 for app, two Abbott kits and online monitoring of test) for our return to the U.S. from Iceland in mid-July. I am hopeful that Iceland Air will accept negative results, shown on that app, and let us board in KEF.
If anyone uses eMed, Optum or any similar service, please report your experience with it here. Thank you, and safe travels.