Eric, it WORKS!!!
On IPN page:
$ATC_Code1 = "".$_POST['option_selection1_'.($displayIndex+1)] ."";
$ATC_Code2 = "".$_POST['option_selection2_'.($displayIndex+1)] ."";
Two things that I did not try at the same time: A) ' single quote marks inside (other values have " two, B) the underscore at the end option_selection1_
After I have also used the first variable to passthrough the product SKU by adding a third variable and it worked.