You're young, two weeks won't matter, but I understand your impatience. Like so many others of us, I've been there, and waited several months longer than I would have liked (seven months instead of the standard three). I think I'm in the minority, I was a good girl, despite how impatient a person I am.
One consideration might be to call her office and ask if she's willing to order the blood work early. There's no real need for you to see your endo herself to get the blood work done; it'll be good for her to have that available when she does meet with you. Further, if you can get the results, you'll be able to see for yourself if things are out of norm. If you've started to self-medicate before you get the results, and anything is out of whack on the report you should probably stop and wait for your endo to go over them. If they're ok, well then, it is unlikely your endo will ever know you started taking them early as your first blood work will be pre-hrt.
It's also very possible that since you're young your endo will write the scrips on the day you see her, in particular if she has many transsexual patients and understands their impatience. She might plan on calling you or changing your prescription only if she sees something wrong with your bloodwork later. If that's the case, there's no real need to order online - it's going to take a couple weeks to get them to you anyway. But, as you've said, at least you'll have them if you want them and she postpones again.
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"Go confidently in the direction of your dreams." - Thoreau