If  you are in the UK, a pre-paid envelope at the 2nd class rate will be delivered at the first class speed if you use a first class stamp.

No.

The pre-paid envelopes are charged to the addressee at a specific amount. 

The addressee may at a later date receive a credit for the amount of the stamp that you added to the envelope.

So you are eventually saving the addressee some money (which is why some nonprofits say your stamp "helps"), but it doesn't affect … Read more