After seven years, it's raising its membership fee.It was the announcement investors had been waiting for. Last week, Costco Wholesale (COST 0.69%) finally said that it would raise its membership fee by $5 starting Sept. 1.That increase isn't likely to affect members. But $5 extra for every single one of Costco's tens of millions of members is a lot of money for the company. Let's see how much, and why investors are greeting this news so enthusiastically.Why raise it now?Costco last raised its fee in 2017, ...