In the U.S., ZRT Labs is a reputable mail-in service. So it is possible to get tested this way. You should try to find some unbiased reviews of the particular lab you'll be using.
As for the blood samples, the hormone levels shouldn't change much after collection until too much time has passed and they start degrading. Any good lab will have run tests to gain an understanding of the maximum time possible between sample collection and testing. Storage conditions matter—it may be that there's less time in the summer months.