First rule - it is usually easier to trim the card size to an envelope than find a suitable envelope to match a card, so if I see a bargain pack of interesting envelopes I go for them.
Here I've used a corner punch to decorate the pink envelope flap, and glued the punch-outs on, to enhance the decoupage lady. I could have done with a bit of bling too to emphasis bits, but where's the bally bling when you need it, eh ? or Googley eyes!

Sets of matching writing paper are a bonus and can be used for inserts, carrying the colour theme through, as below (purple).


The above two (totally mineown) were printed in an early issue of CardMaking and Papercrafts mag, which also does Quick Cards - and they also have some nice free downloads on their website.