If I work 100% remotely for a Florida company but I live in Virginia, do I have to pay Virginia state income tax?

Yes you have to notify your employer. You will have to pay taxes on your Virginia income to Virginia from the start of the year until when your leave the state. You will have to establish residence in Florida to prove to Virginia that you no longer live there.

How much can I claim without receipts?

$300

In order to be eligible for a tax deduction, you are required to present documented documentation if the total amount of your claimed expenses is more than $300. On the other hand, if the entire amount of your claimed expenses is less than $300, you are exempt from the requirement to present receipts.