It is a bit of a trick by ebay.  

To calculate you need to add the $243 plus all the postage costs.

Calculate 9.9% of this total (X).

Then ebay calculate the % as the (X) you just calculated, against your $243 in sales.  This is why you are getting 17.79%

ie:     X/243 x 100

On top of this is the Paypal fees...... (which aren't on your invoice)