There's a bunch of videos showing how to ship with eBay Standard Envelope. I've learned from them and modified my approach based on experience. There's no one right way, but I'll share with you what has worked for me, whether it's 1 card or 12.
Be sure to check out my eBay store: https://www.ebay.com/str/mpscards/
Security Envelopes: https://www.amazon.com/gp/product/B08V36LD4Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Photo Mailers: https://www.amazon.com/gp/product/B00DR15B94/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
eBay Mailers: https://www.ebay.com/itm/256171316683?var=556147057602