I have some large commercial card envelopes that I have acquired over the years that are often too large for the cards I am making. I finally decided to work out a way to use them for my A2 invitation sized cards.
I start out with a 8.25" x 5.75" envelope and cut out the middle section to fit my A2 card, cut off both ends from the scraps and glue them back on each side. You end up with a nice slim envelope with a nice triangular flap. Adding the two side pieces covers up your card and keeps the card from being damaged in transit.
If you have a stash of large envelopes consider modifying them so you can use them for your smaller cards.
I hope the info on the right is useful.
If you have a card shop close to you inquire about getting the envelopes when they change up their card displays. The envelopes are not returned to the company and you can often get them for free. I use them for my personal cards and for the cards I refurbish. I think it would be inappropriate for them to be used for cards made for sale.