Poshmark deducts one commission from each sale and nothing else. Under $15 the fee is a flat $2.95. At $15 and above it is 20% of the sale price, so you keep 80%. Listing is free, and there is no separate payment processing fee.
Buyers pay for the standard shipping label on top of your price, so shipping never touches your earnings or the commission. You only pay shipping when a box weighs more than 5 lb and needs a label upgrade, up to the 15 lb maximum. Enter that upgrade in the label field when it applies.
Because the fee switches from flat to percentage at $15, a price just under the line can earn more than a price on it. A $14.99 sale leaves you $12.04; a $15.00 sale leaves $12.00. Above that point the 20% commission is the number to plan around.
Profit subtracts what you paid for the item and your packaging. Margin is profit divided by the sale price. ROI is profit divided by the item cost, the figure most resellers use to decide how much to pay at a thrift store or estate sale.
Commission = $2.95 if the sale is under $15, otherwise 20% x sale price. Earnings = sale price - commission