It is my understanding that the seller does not lose money if the buyer uses ebay plus -  if you item is offered as free postage in the first place, it makes no difference to the amount you receive

Someone else may know more.

Also, ebay only should put ebay plus on items that have free postage and 30 days returns.    So I never offer free postage or returns

eBay Plus Criteria*

As a seller:

• Above Standard or eBay Top Rated
• Compliant with our selling policies

For your listings:

• Item location and return address in Australia
• Fixed-price format
• Item isn’t heavy or bulky

Postage and returns:

• Free domestic postage**
• No additional postage charge for multiple items
• Accept 30-day+ returns