At 6.50 you're still losing money on ground advantage if it goes very far (I am regularly seeing prices of 6.40 or higher even with the
ebay discounts). And don't forget you pay commission on shipping fees paid to you (and in some states, your customer pays sales tax on the shipping fee, and you pay commission on that tax too).
I have used 6.50 as my ground advantage charge for a while but just decided earlier today that was going to move it to 7.50. I may finally move my $1 first class shipping to $1.25 too.