Just for the record, given you did not share your solution…:
function doPost(e) {
var ss = SpreadsheetApp.getActive();
var sh = ss.getSheetByName('Sheet1');
var nc = sh.getLastColumn() + 1;
var rows = JSON.parse(e.postData.contents);
var irl = rows[0].length;
sh.insertColumns(nc,irl);
for (var i=0;i<rows.length;i++) {
var range = sh.getRange(i+1,nc, 1, irl);
range.setValues([rows[i]]);
}
return ContentService.createTextOutput("Success") ;
}