Help! To charge an application fee or not?

I am new to real estate investment and just bought my first rental property in MA ( a condo).  I am having an "open house" this weekend with multiple showings at different appointment times and expect maybe 10 prospective tenants to show up.  I have a top applicant in mind already based on our phone conversation, as long as everything checks out with the formal application and credit check.  I am wondering if I should just pay the background/credit check fee myself for my top applicant ($35), or if I should collect an application fee from everyone and then refund it to those who don't pass my initial application screening?  I don't want to run numerous checks and run the risk of being resented/accused of discrimination by an applicant who is not selected over $35. I intend to use a website called CoreLogic for tenant screening.  Are there other sites that you would recommend instead? Are there any reputable sites that do credit /background checks for free?