Derek & Sharon shared this inverted security envelope idea on Design Sponge earlier and I thought it was worth mentioning. Now, probably not a great idea for wedding invitations – but it’s a very neat way to dress up thank you notes. They  are very affordable – all you need is a little glue. They are also environmentally friendly, since you are reusing envelopes rather then throwing them out. Visit Design Sponge for simple, easy to follow instructions.