> We recognize, however, that developers will incur costs to support their billing system, so when a user selects alternative billing, we will reduce the developer’s service fee by 4%. For example, for the vast majority of developers who pay 15% for transactions through Google Play's billing system, their service fee for transactions through the alternate billing system would be 11%.
You have to pay a 11% fee for using someone else's billing system when buying an in-app purchase, if that app was downloaded from the Play store?