You may have difficultly getting a 90 day, both the doc and pharmacist have to agree to do it - 3x30 day with 'fill on dates' is more likely.
When I had a long out of town trip I was able to get a 60 day script. When I came back the doc sent 60 day script again but the pharmacist wouldn't fill it and only allowed it because what ever code/note the doc added about long term travel. That was self pay so I wasn't even a risk for selling it. Some states won't allow more than 3x30 day.
I am surprised you got name brand vyvanse for $300/month, generic is ~$250/month without coupons/discount cards