I learned a good way to use the free, USPS triangle tubes. I've used them for plants in cups and bareroot, or bagged roots with soil. You lay the plant and cup or bagged roots (wrapped in paper and plastic) on the unfolded box and poke a hole on each side of the plant, then thread a twisty or string through the holes and tie the plant to the box so it doesnt shift and slide around. Do that in a few places. Then fold up the box and put tape over the exposed twist tie or string, so it doesnt get snagged in transit. I dont know if the box will accomodate a gallon pot, but should fit the roots and soil in a bag. Shipping has been between aprox $5 and $7 If you print the postage shipping lable at home you will save about $1
USPS sml tube L-25” h-6” w-6” g-19” both ssize triangle tubes ship for the same price (what ever the weight is)
USPS med tube L-38” h-4” w-6” g-17”