Calculating true payload capacity

So we all know that a trucks payload capacity isn't the only thing to consider when determining if a truck is within spec of what you plan on towing but it does have importance. So this got me wondering about when does a manufacturer determine what the payload capacity is and are all trucks put on a scale and weighed when they come off the assembly line and then the sticker gets placed on the door jamb after subtracting that number from the GVWR or does one truck get weighed and that payload number goes on stickers of all like vehicles being made? What about if there are factory installed options like a sprayed in bed liner for instance or factory supplied floor mats or maybe you order a bigger alternator or whatever? Does that make that number incorrect or is weighing it the last thing that gets done? Obviously anything a dealer installs lowers the payload number on any sticker and the only true way of knowing is to weigh the truck on a certified scale and subtract that amount from the GVWR. People state in the forums all the time about how the payload is on the sticker on the door jamb and you can't use the brochure advertised number. Well, I don't think you can use that sticker number either. Add a bed liner, tonneau cover, undercoating, running boards, floor mats, hitch and any number of other things and the real number is quite a bit different from what the sticker says. Now try to explain all this and the importance of RAWR and GCWR and tire load ratings etc. to a sales person. Suddenly you realize they have no idea what they're talking about, just like RV sales people! They just want to make a sale.