A small duck is light, and light packages get crushed. The bill, the hat, and any thin prop need air around them and a wall that does not flex onto the face.
Wrap the face
Tissue or a soft wrap around the head, then a small box or a folded card that keeps the wrap from being the only structure. Do not let the bill touch the inside of a mailer with nothing else in between. A bubble mailer alone is a gamble for anything with a point.
If the duck has a separate part, bag that part. Loose pieces in a mailer become lost pieces.
Choose the box from the duck, not from the pile
The box should be a little larger than the wrapped figure, not a shoe box with one duck rolling inside. Empty space needs paper fill. The figure should not click when you shake the closed box.
A rigid mailer can work for a very small, fully wrapped duck. A thin bill still prefers a box.
Label the contents honestly
If you sell it, the outside does not need the theme in giant letters. The inside should match the listing: the duck they bought, not a surprise remix. Include a short note with care if you want, such as “display piece, keep out of hot cars.” Do not include the STL.
Postage
Weigh the packed box. Use that weight, not the weight of the bare duck. Underpaid postage is how a gift arrives late or not at all. Pricing should already include this mailer.
Markets versus mail
A duck you hand to someone at a craft fair can go in a simple bag if they are carrying it home. A duck you ship needs the box. Do not use the fair bag as your mail test.
Print a spare of anything with a fragile bill if the order matters. It is cheaper than reprinting after a damage photo, and it teaches you which orientation survives your own packing.