In proper postage etiquette, the postage stamps is supposed to be affixed to the upper right hand side of the front of the envelope. The address should be in the center of the envelope, and the return address in the upper left hand side.
When addressing a side flapped envelope the side flap should be to to the right - regardless as to whether the person is addressing the envelope is left or right handed. So the postage stamp is on the same side as the opening flap.
Presuming the postage is of the correct amount, and the stamp is entirely on the front of the envelope (not 1/3 on the envelope and 2/3 over the top or side) it should not matter.
The sender address goes either on the back at the top of the envelope or on the left hand corner in the front. The receiver's address goes in the front center of the envelope, and the stamp goes on the top right hand corner of the front.
In generally goes on the upper left corner of the front of the envelope.
The front of the envelope would have 1, while the back would have 2.
material yardage
up down side to side
When sending a wedding invitation the response card should go into the response envelope and you should address it back to you on the front of the response envelope with a stamp on the envelope.
This depends on what is usual in your country. We I live we put the address where the letter is going on the front, left hand side and the return address on the back of the envelope.
With the front of the card facing the outside flap. This way, they see the card immediately upon opening the envelope, and also so the postal scanner won't dent the "good" side of the card. Also, if the card has any money inside it, you may want to put the side that opens up towards the top of the envelope so when you take it out the money comes with it. If you put it the other way round there may be cases when you do not take the money out and naturally you throw away the envelope not realizing the cash inside it!
The back of the envelope will be the triangular-shaped part that folds down in back; it usually has some kind of adhesive to seal the envelope (most often by moistening the strip).