If I am not mistaken, Payload is a different number from Towing capacity.
Payload is how much the Truck can carry. Towing is how much it will PULL. Where the two come together is Tongue Weight which is what needs to get added into the Payload figure along with passengers and all the stuff IN your truck. GCVW (Gross Combined Vehicle Weight) is what the two vehicles Truck and Trailer should not Exceed. But this is not stated on the sticker)
The Sticker on your door only pertains to the truck and it's Payload.
You are much better off as so many have pointed out with a 3/4 ton, WITH a TOWING PACKAGE and CLASS 5 Hitch from the FACTORY. As they say you will not regret it.