Motorway sticker for Czech Republic

We are planning to visit the Czech Republic in May/June and understand that we need to obtain a sticker for our Executive as it is under 3.5t. However our registration document does not show the weight anywhere on it. Does anyone know how we prove that we are below the limit when we go to buy the sticker, especially as the website says that they can be bought at rest areas/post offices or garages.

Any help or advice on the area would be greatly appreciated.

Thanks
Sue and Brian