One requirement for your items to have the Premium Service badging is to offer 30 day money back returns. If that doesn't suit what you sell then you can set your own Returns policies and if you still meet the other requirements you will maintain your TRS.

From what I've read TRS listings with or without the Premium badge have better placement in Best Match search rankings over non-TRS listings. And it shouldn't make any difference to search results whether you have a store or not.

More info

http://sellercentre.ebay.com.au/ebay_for_business/top_rated