Margin is different from markup
Profit margin means profit as a share of selling price, not cost. Selling fees here are also a share of price. Include packaging, shipping you absorb, and allocated overhead in full unit cost.
Example and limitations
With cost 60, margin 30% and fees 5%, calculated price is about 92.3077. After fees and cost, profit is 30% of price. If you round the price, recheck the resulting margin.
Does it add VAT?
No. Decide whether inputs include tax and use the VAT calculator separately when needed.