Update: Mercari has announced they will be postponing their transition to a 10% fee environment until November 21, 2016. This is their formal statement emailed to all subscribers/users. (Note that all comments below still remain true, just delayed one month.)
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.