Here is an envelope mini that I built from left over invitation envelopes from my wedding (seventeen years ago). They were still pristine white so I figure they are acid-free after all this time!
Please forgive the photo quality - I'm a crafter not a photographer but I promise to get better!
The envelopes were made from a white paper embossed with flowers and leaves. I inked them with a pink pad, intending this to be a baby girl photo display. I made all of the double mats from scrap paper I had on hand and then added some additional embellishments made from the same scrap paper. Finally, I added rhinestones, crystals, ribbon, silk/felt flowers and metal fasteners from EK Success.