By default, Poshmark produces labels in an 8.5 x 11 size which won't fit correctly 4 x 6 labels which is iDPRT shipping label printer default label size.

So you need to update your label size setting on Posh Mark to 4 x 6.

The steps: To select your shipping label size, go to: Account Tab > My Seller tools > Shipping Label Settings

Please refer to the below link for the detailed instruction to change the label size:

https://support.poshmark.com/s/article/945594407?language=en_US