I've spent a lot of time trying to find an inexpensive and quality solution - here it is. I can put them in a picture envelope, mail them in a plastic bag, and they'll arrive safe and sound (assuming the USPS doesn't decide to consider my "fragile" stickers a silly personal challenge).
BOX USA BSP1814 Corrugated Sheets: Premium Packaging & Shipping Protection
3 Review
π¦ Aviditi SF18 Single Corrugated Length Box
3 Review
Drive Medical M6026 Convoluted Foam Mattress Pad - White
3 Review
π¦ Juvale Corrugated Cardboard Sheets: A Versatile Solution for Packaging & Shipping Supplies
3 Review