Whether you sell on Poshmark, Mercari, or some other app, the shipping is almost universally the same.
On Poshmark, every item, no matter the size, weight, or type, costs $5.95 to ship. That really eats into your profit margin.
On Mercari, the same is pretty much true. For items weighing 8 oz. or less, you can use Mercari’s minimum $4 shipping. If your item weighs between 8-16 oz., the only option they offer is $6 priority shipping.
That’s why I always ship it myself.