You buy an item for 10 including shipping.

Typical retail margin in your industry is 50%.

So you should sell it for 15.

Wrong.

The only thing that matters is how much it is worth to your customers. If they are willing to pay 20 or 25 for it, why sell cheaper unless you have a reason to?

Always applying a multiplier is not a pricing strategy, it is a primary school maths question.